diff --git a/include/asm/symbol.h b/include/asm/symbol.h
index 6ed35c67b..729ec28f3 100644
--- a/include/asm/symbol.h
+++ b/include/asm/symbol.h
@@ -120,6 +120,7 @@ struct Symbol *sym_AddAnonLabel(void);
 void sym_WriteAnonLabelName(char buf[MIN_NB_ELMS(MAXSYMLEN + 1)], uint32_t ofs, bool neg);
 void sym_Export(char const *symName);
 struct Symbol *sym_AddEqu(char const *symName, int32_t value);
+struct Symbol *sym_RedefEqu(char const *symName, int32_t value);
 struct Symbol *sym_AddSet(char const *symName, int32_t value);
 uint32_t sym_GetPCValue(void);
 uint32_t sym_GetConstantSymValue(struct Symbol const *sym);
diff --git a/src/asm/parser.y b/src/asm/parser.y
index 9423ea1f0..31d7feab2 100644
--- a/src/asm/parser.y
+++ b/src/asm/parser.y
@@ -851,6 +851,7 @@ directive	: endc
 		| warn
 		| assert
 		| def_equ
+		| redef_equ
 		| def_set
 		| def_rb
 		| def_rw
@@ -1118,6 +1119,11 @@ def_equ		: def_id T_POP_EQU const {
 		}
 ;
 
+redef_equ	: redef_id T_POP_EQU const {
+			sym_RedefEqu($1, $3);
+		}
+;
+
 def_set		: def_id set_or_equal const {
 			sym_AddSet($1, $3);
 		}
diff --git a/src/asm/rgbasm.5 b/src/asm/rgbasm.5
index 086206061..f21a72ebf 100644
--- a/src/asm/rgbasm.5
+++ b/src/asm/rgbasm.5
@@ -967,6 +967,26 @@ def SCREEN_HEIGHT equ 144
 Note that colons
 .Ql \&:
 following the name are not allowed.
+.Pp
+If you
+.Em really
+need to, the
+.Ic REDEF
+keyword will define or redefine a constant symbol.
+This can be used, for example, to update a constant using a macro, without making it mutable in general.
+.Bd -literal -offset indent
+    def NUM_ITEMS equ 0
+MACRO add_item
+    redef NUM_ITEMS equ NUM_ITEMS + 1
+    def ITEM_{02x:NUM_ITEMS} equ \1
+ENDM
+    add_item 1
+    add_item 4
+    add_item 9
+    add_item 16
+    assert NUM_ITEMS == 4
+    assert ITEM_04 == 16
+.Ed
 .Ss Mutable constants
 .Ic SET ,
 or its synonym
diff --git a/src/asm/symbol.c b/src/asm/symbol.c
index 3982e4722..1c70b04ea 100644
--- a/src/asm/symbol.c
+++ b/src/asm/symbol.c
@@ -397,6 +397,30 @@ struct Symbol *sym_AddEqu(char const *symName, int32_t value)
 	return sym;
 }
 
+struct Symbol *sym_RedefEqu(char const *symName, int32_t value)
+{
+	struct Symbol *sym = sym_FindExactSymbol(symName);
+
+	if (!sym)
+		return sym_AddEqu(symName, value);
+
+	if (sym_IsDefined(sym) && sym->type != SYM_EQU) {
+		error("'%s' already defined as non-EQU at ", symName);
+		dumpFilename(sym);
+		putc('\n', stderr);
+		return NULL;
+	} else if (sym->isBuiltin) {
+		error("Built-in symbol '%s' cannot be redefined\n", symName);
+		return NULL;
+	}
+
+	updateSymbolFilename(sym);
+	sym->type = SYM_EQU;
+	sym->value = value;
+
+	return sym;
+}
+
 /*
  * Add a string equated symbol.
  *
diff --git a/test/asm/def.asm b/test/asm/def.asm
index 4e896c709..076113de6 100644
--- a/test/asm/def.asm
+++ b/test/asm/def.asm
@@ -26,4 +26,5 @@ def constant equ 6*7 ; fails
 redef string equs "there"
 	println "{string}"
 
-redef constant equ 6*9 ; syntax error
+redef constant equ 6*9
+	println constant
diff --git a/test/asm/def.err b/test/asm/def.err
index 2fe3ac940..29812c306 100644
--- a/test/asm/def.err
+++ b/test/asm/def.err
@@ -1,5 +1,3 @@
 ERROR: def.asm(23):
     'constant' already defined at def.asm(10)
-ERROR: def.asm(29):
-    syntax error, unexpected EQU, expecting SET or = or EQUS
-error: Assembly aborted (2 errors)!
+error: Assembly aborted (1 error)!
diff --git a/test/asm/def.out b/test/asm/def.out
index e09b8df47..0ecc9660a 100644
--- a/test/asm/def.out
+++ b/test/asm/def.out
@@ -7,3 +7,4 @@ here
 $0 $1 $5 $9
 $2A
 there
+$36
diff --git a/test/asm/redef-equ.asm b/test/asm/redef-equ.asm
new file mode 100644
index 000000000..1598d43ae
--- /dev/null
+++ b/test/asm/redef-equ.asm
@@ -0,0 +1,23 @@
+DEF n EQU 0
+REDEF n EQU 1
+; prints "$1"
+PRINTLN n
+
+list: MACRO
+LIST_NAME EQUS "\1"
+DEF LENGTH_{LIST_NAME} EQU 0
+ENDM
+
+item: MACRO
+REDEF LENGTH_{LIST_NAME} EQU LENGTH_{LIST_NAME} + 1
+DEF {LIST_NAME}_{d:LENGTH_{LIST_NAME}} EQU \1
+ENDM
+
+	list SQUARES
+	item 1
+	item 4
+	item 9
+	println LENGTH_SQUARES, SQUARES_1, SQUARES_2, SQUARES_3
+
+N EQUS "X"
+REDEF N EQU 42
diff --git a/test/asm/redef-equ.err b/test/asm/redef-equ.err
new file mode 100644
index 000000000..5dce53567
--- /dev/null
+++ b/test/asm/redef-equ.err
@@ -0,0 +1,3 @@
+ERROR: redef-equ.asm(23):
+    'N' already defined as non-EQU at redef-equ.asm(22)
+error: Assembly aborted (1 error)!
diff --git a/test/asm/redef-equ.out b/test/asm/redef-equ.out
new file mode 100644
index 000000000..aec9ed58f
--- /dev/null
+++ b/test/asm/redef-equ.out
@@ -0,0 +1,2 @@
+$1
+$3$1$4$9
diff --git a/test/asm/reference-undefined-equs.asm b/test/asm/reference-undefined-equs.asm
new file mode 100644
index 000000000..aaed3bd46
--- /dev/null
+++ b/test/asm/reference-undefined-equs.asm
@@ -0,0 +1,5 @@
+SECTION "sec", ROM0[0]
+	db s1, s2
+
+def s1 equs "1"
+redef s2 equs "2"
diff --git a/test/asm/reference-undefined-equs.err b/test/asm/reference-undefined-equs.err
new file mode 100644
index 000000000..b2657d06a
--- /dev/null
+++ b/test/asm/reference-undefined-equs.err
@@ -0,0 +1,5 @@
+ERROR: reference-undefined-equs.asm(4):
+    's1' already referenced at reference-undefined-equs.asm(2)
+ERROR: reference-undefined-equs.asm(5):
+    's2' already referenced at reference-undefined-equs.asm(2)
+error: Assembly aborted (2 errors)!
diff --git a/test/asm/reference-undefined-equs.out b/test/asm/reference-undefined-equs.out
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/asm/reference-undefined-sym.asm b/test/asm/reference-undefined-sym.asm
index e12c22747..06650528b 100644
--- a/test/asm/reference-undefined-sym.asm
+++ b/test/asm/reference-undefined-sym.asm
@@ -1,4 +1,7 @@
 SECTION "sec", ROM0[0]
-	db X
+	db x1, x2, y1, y2
 
-X = 2
+def x1 = 1
+redef x2 = 2
+def y1 equ 3
+redef y2 equ 4
diff --git a/test/asm/reference-undefined-sym.out.bin b/test/asm/reference-undefined-sym.out.bin
index e69de29bb..82090ee2c 100644
--- a/test/asm/reference-undefined-sym.out.bin
+++ b/test/asm/reference-undefined-sym.out.bin
@@ -0,0 +1 @@
+

\ No newline at end of file