diff --git a/hasher.go b/hasher.go
index 5dd64704..cb5b3fb8 100644
--- a/hasher.go
+++ b/hasher.go
@@ -14,7 +14,7 @@ const (
 	NodePrefix = 1
 )
 
-var _ hash.Hash = (*Hasher)(nil)
+var _ hash.Hash = (*NmtHasher)(nil)
 
 var (
 	ErrUnorderedSiblings         = errors.New("NMT sibling nodes should be ordered lexicographically by namespace IDs")
@@ -23,7 +23,24 @@ var (
 	ErrInvalidNodeNamespaceOrder = errors.New("invalid NMT node namespace order")
 )
 
-type Hasher struct {
+// Hasher describes the interface nmts use to hash leafs and nodes.
+//
+// Note: it is not advised to create alternative hashers if following the
+// specification is desired. The main reason this exists is to not follow the
+// specification for testing purposes.
+type Hasher interface {
+	IsMaxNamespaceIDIgnored() bool
+	NamespaceSize() namespace.IDSize
+	HashLeaf(data []byte) ([]byte, error)
+	HashNode(leftChild, rightChild []byte) ([]byte, error)
+	EmptyRoot() []byte
+}
+
+var _ Hasher = &NmtHasher{}
+
+// NmtHasher is the default hasher. It follows the description of the original
+// hashing function described in the LazyLedger white paper.
+type NmtHasher struct { //nolint:revive
 	baseHasher   hash.Hash
 	NamespaceLen namespace.IDSize
 
@@ -41,16 +58,16 @@ type Hasher struct {
 	data []byte // written data of the NMT node
 }
 
-func (n *Hasher) IsMaxNamespaceIDIgnored() bool {
+func (n *NmtHasher) IsMaxNamespaceIDIgnored() bool {
 	return n.ignoreMaxNs
 }
 
-func (n *Hasher) NamespaceSize() namespace.IDSize {
+func (n *NmtHasher) NamespaceSize() namespace.IDSize {
 	return n.NamespaceLen
 }
 
-func NewNmtHasher(baseHasher hash.Hash, nidLen namespace.IDSize, ignoreMaxNamespace bool) *Hasher {
-	return &Hasher{
+func NewNmtHasher(baseHasher hash.Hash, nidLen namespace.IDSize, ignoreMaxNamespace bool) *NmtHasher {
+	return &NmtHasher{
 		baseHasher:       baseHasher,
 		NamespaceLen:     nidLen,
 		ignoreMaxNs:      ignoreMaxNamespace,
@@ -59,7 +76,7 @@ func NewNmtHasher(baseHasher hash.Hash, nidLen namespace.IDSize, ignoreMaxNamesp
 }
 
 // Size returns the number of bytes Sum will return.
-func (n *Hasher) Size() int {
+func (n *NmtHasher) Size() int {
 	return n.baseHasher.Size() + int(n.NamespaceLen)*2
 }
 
@@ -69,7 +86,7 @@ func (n *Hasher) Size() int {
 // write is allowed.
 // It panics if more than one single write is attempted.
 // If the data does not match the format of an NMT non-leaf node or leaf node, an error will be returned.
-func (n *Hasher) Write(data []byte) (int, error) {
+func (n *NmtHasher) Write(data []byte) (int, error) {
 	if n.data != nil {
 		panic("only a single Write is allowed")
 	}
@@ -101,7 +118,7 @@ func (n *Hasher) Write(data []byte) (int, error) {
 // Sum computes the hash. Does not append the given suffix, violating the
 // interface.
 // It may panic if the data being hashed is invalid. This should never happen since the Write method refuses an invalid data and errors out.
-func (n *Hasher) Sum([]byte) []byte {
+func (n *NmtHasher) Sum([]byte) []byte {
 	switch n.tp {
 	case LeafPrefix:
 		res, err := n.HashLeaf(n.data)
@@ -125,17 +142,17 @@ func (n *Hasher) Sum([]byte) []byte {
 }
 
 // Reset resets the Hash to its initial state.
-func (n *Hasher) Reset() {
+func (n *NmtHasher) Reset() {
 	n.tp, n.data = 255, nil // reset with an invalid node type, as zero value is a valid Leaf
 	n.baseHasher.Reset()
 }
 
 // BlockSize returns the hash's underlying block size.
-func (n *Hasher) BlockSize() int {
+func (n *NmtHasher) BlockSize() int {
 	return n.baseHasher.BlockSize()
 }
 
-func (n *Hasher) EmptyRoot() []byte {
+func (n *NmtHasher) EmptyRoot() []byte {
 	n.baseHasher.Reset()
 	emptyNs := bytes.Repeat([]byte{0}, int(n.NamespaceLen))
 	h := n.baseHasher.Sum(nil)
@@ -145,7 +162,7 @@ func (n *Hasher) EmptyRoot() []byte {
 }
 
 // ValidateLeaf verifies if data is namespaced and returns an error if not.
-func (n *Hasher) ValidateLeaf(data []byte) (err error) {
+func (n *NmtHasher) ValidateLeaf(data []byte) (err error) {
 	nidSize := int(n.NamespaceSize())
 	lenData := len(data)
 	if lenData < nidSize {
@@ -160,7 +177,7 @@ func (n *Hasher) ValidateLeaf(data []byte) (err error) {
 // leaves minNs = maxNs = ns(leaf) = leaf[:NamespaceLen]. HashLeaf can return the ErrInvalidNodeLen error if the input is not namespaced.
 //
 //nolint:errcheck
-func (n *Hasher) HashLeaf(ndata []byte) ([]byte, error) {
+func (n *NmtHasher) HashLeaf(ndata []byte) ([]byte, error) {
 	h := n.baseHasher
 	h.Reset()
 
@@ -187,7 +204,7 @@ func (n *Hasher) HashLeaf(ndata []byte) ([]byte, error) {
 
 // MustHashLeaf is a wrapper around HashLeaf that panics if an error is
 // encountered. The ndata must be a valid leaf node.
-func (n *Hasher) MustHashLeaf(ndata []byte) []byte {
+func (n *NmtHasher) MustHashLeaf(ndata []byte) []byte {
 	res, err := n.HashLeaf(ndata)
 	if err != nil {
 		panic(err)
@@ -197,7 +214,7 @@ func (n *Hasher) MustHashLeaf(ndata []byte) []byte {
 
 // ValidateNodeFormat checks whether the supplied node conforms to the
 // namespaced hash format and returns ErrInvalidNodeLen if not.
-func (n *Hasher) ValidateNodeFormat(node []byte) (err error) {
+func (n *NmtHasher) ValidateNodeFormat(node []byte) (err error) {
 	expectedNodeLen := n.Size()
 	nodeLen := len(node)
 	if nodeLen != expectedNodeLen {
@@ -216,7 +233,7 @@ func (n *Hasher) ValidateNodeFormat(node []byte) (err error) {
 // nodes in an NMT have correct namespace IDs relative to each other, more
 // specifically, the maximum namespace ID of the left sibling should not exceed
 // the minimum namespace ID of the right sibling. It returns ErrUnorderedSiblings error if the check fails.
-func (n *Hasher) validateSiblingsNamespaceOrder(left, right []byte) (err error) {
+func (n *NmtHasher) validateSiblingsNamespaceOrder(left, right []byte) (err error) {
 	if err := n.ValidateNodeFormat(left); err != nil {
 		return fmt.Errorf("%w: left node does not match the namesapce hash format", err)
 	}
@@ -237,7 +254,7 @@ func (n *Hasher) validateSiblingsNamespaceOrder(left, right []byte) (err error)
 // validity of the inputs of HashNode. It verifies whether left
 // and right comply by the namespace hash format, and are correctly ordered
 // according to their namespace IDs.
-func (n *Hasher) ValidateNodes(left, right []byte) error {
+func (n *NmtHasher) ValidateNodes(left, right []byte) error {
 	if err := n.ValidateNodeFormat(left); err != nil {
 		return err
 	}
@@ -260,7 +277,7 @@ func (n *Hasher) ValidateNodes(left, right []byte) error {
 // slightly changes. Let MAXNID be the maximum possible namespace ID value i.e., 2^NamespaceIDSize-1.
 // If the namespace range of the right child is start=end=MAXNID, indicating that it represents the root of a subtree whose leaves all have the namespace ID of `MAXNID`, then exclude the right child from the namespace range calculation. Instead,
 // assign the namespace range of the left child as the parent's namespace range.
-func (n *Hasher) HashNode(left, right []byte) ([]byte, error) {
+func (n *NmtHasher) HashNode(left, right []byte) ([]byte, error) {
 	// validate the inputs
 	if err := n.ValidateNodes(left, right); err != nil {
 		return nil, err
diff --git a/hasher_test.go b/hasher_test.go
index 8227ed3e..0bf1178e 100644
--- a/hasher_test.go
+++ b/hasher_test.go
@@ -600,7 +600,7 @@ func TestWrite_Err(t *testing.T) {
 
 	tests := []struct {
 		name    string
-		hasher  *Hasher
+		hasher  *NmtHasher
 		data    []byte
 		wantErr bool
 		errType error
@@ -639,7 +639,7 @@ func TestSum_Err(t *testing.T) {
 
 	tests := []struct {
 		name         string
-		hasher       *Hasher
+		hasher       *NmtHasher
 		data         []byte
 		nodeType     byte
 		wantWriteErr bool
diff --git a/nmt.go b/nmt.go
index 415e7247..5b897f00 100644
--- a/nmt.go
+++ b/nmt.go
@@ -39,6 +39,7 @@ type Options struct {
 	// in the "Hasher.
 	IgnoreMaxNamespace bool
 	NodeVisitor        NodeVisitorFn
+	Hasher             Hasher
 }
 
 type Option func(*Options)
@@ -82,8 +83,15 @@ func NodeVisitor(nodeVisitorFn NodeVisitorFn) Option {
 	}
 }
 
+// CustomHasher replaces the default hasher.
+func CustomHasher(h Hasher) Option {
+	return func(o *Options) {
+		o.Hasher = h
+	}
+}
+
 type NamespacedMerkleTree struct {
-	treeHasher *Hasher
+	treeHasher Hasher
 	visit      NodeVisitorFn
 
 	// just cache stuff until we pass in a store and keep all nodes in there
@@ -128,9 +136,18 @@ func New(h hash.Hash, setters ...Option) *NamespacedMerkleTree {
 	for _, setter := range setters {
 		setter(opts)
 	}
-	treeHasher := NewNmtHasher(h, opts.NamespaceIDSize, opts.IgnoreMaxNamespace)
+
+	// first create the default hasher using the updated options
+	hasher := NewNmtHasher(h, opts.NamespaceIDSize, opts.IgnoreMaxNamespace)
+	opts.Hasher = hasher
+
+	// set the options a second time to replace the hasher if needed
+	for _, setter := range setters {
+		setter(opts)
+	}
+
 	return &NamespacedMerkleTree{
-		treeHasher:      treeHasher,
+		treeHasher:      opts.Hasher,
 		visit:           opts.NodeVisitor,
 		leaves:          make([][]byte, 0, opts.InitialCapacity),
 		leafHashes:      make([][]byte, 0, opts.InitialCapacity),
@@ -491,6 +508,27 @@ func (n *NamespacedMerkleTree) MaxNamespace() (namespace.ID, error) {
 	return MaxNamespace(r, n.NamespaceSize()), nil
 }
 
+// ForceAddLeaf adds a namespaced data to the tree without validating its
+// namespace ID. This method should only be used by tests that are attempting to
+// create out of order trees. The default hasher will fail for trees that are
+// out of order.
+func (n *NamespacedMerkleTree) ForceAddLeaf(leaf namespace.PrefixedData) error {
+	nID := namespace.ID(leaf[:n.NamespaceSize()])
+	// compute the leaf hash
+	res, err := n.treeHasher.HashLeaf(leaf)
+	if err != nil {
+		return err
+	}
+
+	// update relevant "caches":
+	n.leaves = append(n.leaves, leaf)
+	n.leafHashes = append(n.leafHashes, res)
+	n.updateNamespaceRanges()
+	n.updateMinMaxID(nID)
+	n.rawRoot = nil
+	return nil
+}
+
 // computeRoot calculates the namespace Merkle root for a tree/sub-tree that
 // encompasses the leaves within the range of [start, end).
 // Any errors returned by this method are irrecoverable and indicate an illegal state of the tree (n).
diff --git a/nmt_test.go b/nmt_test.go
index b1acac27..c4907b5b 100644
--- a/nmt_test.go
+++ b/nmt_test.go
@@ -553,6 +553,19 @@ func TestNodeVisitor(t *testing.T) {
 	}
 }
 
+func TestCustomHasher(t *testing.T) {
+	type customHasher struct {
+		*NmtHasher
+	}
+
+	h := customHasher{NewNmtHasher(sha256.New(), namespace.IDSize(8), true)}
+
+	tree := New(sha256.New(), NamespaceIDSize(8), IgnoreMaxNamespace(true), CustomHasher(h))
+
+	_, ok := tree.treeHasher.(customHasher)
+	require.True(t, ok)
+}
+
 func TestNamespacedMerkleTree_ProveErrors(t *testing.T) {
 	tests := []struct {
 		name     string
@@ -1138,3 +1151,19 @@ func TestEmptyRoot_NMT(t *testing.T) {
 
 	assert.True(t, bytes.Equal(gotEmptyRoot, expectedEmptyRoot))
 }
+
+func TestForcedOutOfOrderNamespacedMerkleTree(t *testing.T) {
+	data := [][]byte{
+		append(namespace.ID{0}, []byte("leaf_0")...),
+		append(namespace.ID{2}, []byte("leaf_1")...),
+		append(namespace.ID{1}, []byte("leaf_2")...),
+		append(namespace.ID{1}, []byte("leaf_3")...),
+	}
+	nidSize := 1
+	tree := New(sha256.New(), NamespaceIDSize(nidSize))
+
+	for _, d := range data {
+		err := tree.ForceAddLeaf(d)
+		assert.NoError(t, err)
+	}
+}
diff --git a/proof.go b/proof.go
index 2db25890..2a1037cf 100644
--- a/proof.go
+++ b/proof.go
@@ -228,7 +228,7 @@ func (proof Proof) VerifyNamespace(h hash.Hash, nID namespace.ID, leaves [][]byt
 // the completeness of the proof by verifying that there is no leaf in the
 // tree represented by the root parameter that matches the namespace ID nID
 // outside the leafHashes list.
-func (proof Proof) VerifyLeafHashes(nth *Hasher, verifyCompleteness bool, nID namespace.ID, leafHashes [][]byte, root []byte) (bool, error) {
+func (proof Proof) VerifyLeafHashes(nth *NmtHasher, verifyCompleteness bool, nID namespace.ID, leafHashes [][]byte, root []byte) (bool, error) {
 	// check that the proof range is valid
 	if proof.Start() < 0 || proof.Start() >= proof.End() {
 		return false, fmt.Errorf("proof range [proof.start=%d, proof.end=%d) is not valid: %w", proof.Start(), proof.End(), ErrInvalidRange)
diff --git a/proof_test.go b/proof_test.go
index 291de098..df0f90fa 100644
--- a/proof_test.go
+++ b/proof_test.go
@@ -276,7 +276,8 @@ func TestVerifyLeafHashes_Err(t *testing.T) {
 	// create a sample tree
 	nameIDSize := 2
 	nmt := exampleNMT(nameIDSize, true, 1, 2, 3, 4, 5, 6, 7, 8)
-	hasher := nmt.treeHasher
+	nmthasher := nmt.treeHasher
+	hasher := nmthasher.(*NmtHasher)
 	root, err := nmt.Root()
 	require.NoError(t, err)
 
@@ -330,7 +331,7 @@ func TestVerifyLeafHashes_Err(t *testing.T) {
 	tests := []struct {
 		name               string
 		proof              Proof
-		Hasher             *Hasher
+		Hasher             *NmtHasher
 		verifyCompleteness bool
 		nID                namespace.ID
 		leafHashes         [][]byte