diff --git a/cpp/src/arrow/type_fwd.h b/cpp/src/arrow/type_fwd.h
index 5deec06c42a9a..37ae9087ded5e 100644
--- a/cpp/src/arrow/type_fwd.h
+++ b/cpp/src/arrow/type_fwd.h
@@ -149,7 +149,7 @@ class Decimal##width;           \
 class Decimal##width##Type;     \
 class Decimal##width##Array;    \
 class Decimal##width##Builder;  \
-class Decimal##width##Scalar;
+struct Decimal##width##Scalar;
 
 DECIMAL_DECL(128)
 DECIMAL_DECL(256)