diff --git a/src/adjtrans.jl b/src/adjtrans.jl
index d5dbe76d..a34e53c2 100644
--- a/src/adjtrans.jl
+++ b/src/adjtrans.jl
@@ -141,6 +141,25 @@ function mul!(
   conj!(res)
 end
 
+function mul!(
+  res::AbstractMatrix,
+  op::AdjointLinearOperator{T, S},
+  m::AbstractMatrix,
+  α,
+  β,
+) where {T, S}
+  p = op.parent
+  (size(m, 1) == size(p, 1) && size(res, 1) == size(p, 2) && size(m, 2) == size(res, 2)) ||
+    throw(LinearOperatorException("shape mismatch"))
+  if ishermitian(p)
+    return mul!(res, p, m, α, β)
+  elseif p.ctprod! !== nothing
+    return p.ctprod!(res, m, α, β)
+  else
+    error("Not implemented")
+  end
+end
+
 function mul!(
   res::AbstractVector,
   op::TransposeLinearOperator{T, S},
@@ -188,6 +207,25 @@ function mul!(
   conj!(res)
 end
 
+function mul!(
+  res::AbstractMatrix,
+  op::TransposeLinearOperator{T, S},
+  m::AbstractMatrix,
+  α,
+  β,
+) where {T, S}
+  p = op.parent
+  (size(m, 1) == size(p, 1) && size(res, 1) == size(p, 2) && size(m, 2) == size(res, 2)) ||
+    throw(LinearOperatorException("shape mismatch"))
+  if issymmetric(p)
+    return mul!(res, p, m, α, β)
+  elseif p.tprod! !== nothing
+    return p.tprod!(res, m, α, β)
+  else
+    error("Not implemented")
+  end
+end
+
 function mul!(
   res::AbstractVector,
   op::ConjugateLinearOperator{T, S},
@@ -200,6 +238,18 @@ function mul!(
   conj!(res)
 end
 
+function mul!(
+  res::AbstractMatrix,
+  op::ConjugateLinearOperator{T, S},
+  v::AbstractMatrix,
+  α,
+  β,
+) where {T, S}
+  p = op.parent
+  mul!(res, p, conj.(v), α, β)
+  conj!(res)
+end
+
 -(op::AdjointLinearOperator) = adjoint(-op.parent)
 -(op::TransposeLinearOperator) = transpose(-op.parent)
 -(op::ConjugateLinearOperator) = conj(-op.parent)
diff --git a/src/constructors.jl b/src/constructors.jl
index 41030e3e..c41852d4 100644
--- a/src/constructors.jl
+++ b/src/constructors.jl
@@ -102,6 +102,8 @@ op = LinearOperator(Float64, 2, 2, false, false,
                     (res, v) -> mul!(res, A, v),
                     (res, w) -> mul!(res, A', w))
 ```
+
+The 3-args `mul!` also works when applying the operator on a matrix.
 """
 function LinearOperator(
   ::Type{T},
diff --git a/src/operations.jl b/src/operations.jl
index ca3d2584..18259d40 100644
--- a/src/operations.jl
+++ b/src/operations.jl
@@ -32,7 +32,11 @@ function mul!(res::AbstractVector, op::AbstractLinearOperator{T}, v::AbstractVec
   end
 end
 
-function mul!(res::AbstractVector, op::AbstractLinearOperator, v::AbstractVector{T}) where {T}
+function mul!(res::AbstractMatrix, op::AbstractLinearOperator, m::AbstractMatrix{T}, α, β) where {T}
+  op.prod!(res, m, α, β)
+end
+
+function mul!(res::AbstractVecOrMat, op::AbstractLinearOperator, v::AbstractVecOrMat{T}) where {T}
   mul!(res, op, v, one(T), zero(T))
 end
 
@@ -73,6 +77,26 @@ function *(
   return v_wrapper(res)
 end
 
+# Apply an operator to a matrix (only in-place, since operator * matrix is a matrix).
+
+function mul!(
+  res::Adjoint{S1, M1},
+  m::Adjoint{S2, M2},
+  op::AbstractLinearOperator{T},
+) where {T, S1, S2, M1 <: AbstractMatrix{S1}, M2 <: AbstractMatrix{S2}}
+  mul!(adjoint(res), adjoint(op), adjoint(m))
+  return res
+end
+
+function mul!(
+  res::Transpose{S1, M1},
+  m::Transpose{S2, M2},
+  op::AbstractLinearOperator{T},
+) where {T, S1, S2, M1 <: AbstractMatrix{S1}, M2 <: AbstractMatrix{S2}}
+  mul!(transpose(res), transpose(op), transpose(m))
+  return res
+end
+
 # Unary operations.
 +(op::AbstractLinearOperator) = op
 
@@ -95,11 +119,11 @@ function -(op::AbstractLinearOperator{T}) where {T}
 end
 
 function prod_op!(
-  res::AbstractVector,
+  res::AbstractVecOrMat,
   op1::AbstractLinearOperator,
   op2::AbstractLinearOperator,
-  vtmp::AbstractVector,
-  v::AbstractVector,
+  vtmp::AbstractVecOrMat,
+  v::AbstractVecOrMat,
   α,
   β,
 )
@@ -162,10 +186,10 @@ end
 # Operator + operator.
 
 function sum_prod!(
-  res::AbstractVector,
+  res::AbstractVecOrMat,
   op1::AbstractLinearOperator,
   op2::AbstractLinearOperator{T},
-  v::AbstractVector,
+  v::AbstractVecOrMat,
   α,
   β,
 ) where {T}
diff --git a/test/test_linop.jl b/test/test_linop.jl
index 64b666c6..71039448 100644
--- a/test/test_linop.jl
+++ b/test/test_linop.jl
@@ -61,6 +61,18 @@ function test_linop()
         @test(norm(transpose(u) * A - transpose(u) * op) <= rtol * norm(u))
         @test(typeof(u' * op * v) <: Number)
         @test(norm(u' * A * v - u' * op * v) <= rtol * norm(u))
+
+        mv = hcat(v, -2v)
+        mu = hcat(u, -2u)
+        res_mat = similar(mu)
+        res_trans = similar(mv)
+        res_adj = similar(mv)
+        mul!(res_mat, op, mv)
+        mul!(res_trans, transpose(op), mu)
+        mul!(res_adj, op', mu)
+        @test(norm(A * mv - res_mat) <= rtol * norm(mv))
+        @test(norm(transpose(A) * mu - res_trans) <= rtol * norm(mu))
+        @test(norm(A' * mu - res_adj) <= rtol * norm(mu))
       end
 
       A3 = Hermitian(A2' * A2)