Water blue, also known as aniline blue, Acid blue 22, Soluble Blue 3M, Marine Blue V, or C.I. 42755, is a chemical compound used as a stain in histology. Water blue stains collagen blue in tissue sections. It is soluble in water and slightly soluble in ethanol.

Water blue's molecular formula is C₃₂H₂₅N₃O₉S₃Na₂. Its CAS number is [28631-66-5] and its SMILES structure is Cc1cc(cc(OS([O-])=O)c1N)/C(=C2C=CC(C=C2)=[N/H+]c3ccc(OS([O-])=O)cc3)c4ccc(Nc5ccc(OS([O-])=O)cc5)cc4.

Water blue is also available in mixture with methyl blue, under name Aniline Blue WS, Aniline blue, China blue, or Soluble blue.

It can be used in the Mallory's connective tissue stain and Gomori's one-step trichrome stain. It is used in differential staining.