The WW domain, (also known as the rsp5-domain or WWP repeating motif) is a modular protein domain that mediates specific interactions with protein ligands. This domain is found in a number of unrelated signaling and structural proteins and may be repeated up to four times in some proteins.

Apart from binding preferentially to proteins that are proline-rich, with particular proline-motifs, [AP]-P-P-[AP]-Y, some WW domains bind to phosphoserine- phosphothreonine-containing motifs.
----
The first structure of the WW domain was determined in solution by NMR approach.

It represented the WW domain of human YAP in complex with peptide ligand containing Proline-Proline-x–Tyrosine (PPxY where x = any amino acid) consensus motif.

The YAP WW domain structure in complex with SMAD-derived, PPxY motif-containing peptide was further refined.

Apart from the PPxY motif, certain WW domains recognize LPxY motif (where L is Leucine), and several WW domains bind to phospho-Serine-Proline (p-SP) or phospho-Threonine-Proline (p-TP) motifs in a phospho-dependent manner.

Structures of these WW domain complexes confirmed molecular details of phosphorylation-regulated interactions.

There are also WW domains that interact with polyprolines that are flanked by arginine residues or interrupted by leucine residues, but they do not contain aromatic amino acids.