In eukaryotes, different chromatin states facilitate or repress gene expression and restrict the activity of transposable elements. Post-translational modifications (PTMs) of amino acid residues on the N-terminal tails of histones are suggested to define such states. The histone lysine methyltransferase (HKMTase) SU(VAR)3-9 RELATED4 (SUVR4) of Arabidopsis thaliana functions as a repressor of transposon activity. Binding of ubiquitin by the WIYLD domain facilitates the addition of two methyl groups to monomethylated lysine 9 of histone H3. By using nuclear magnetic resonance (NMR) spectroscopy, we identified SUVR4 WIYLD (S4WIYLD) as a domain with a four-helix bundle structure, in contrast to three-helix bundles of other ubiquitin binding domains. NMR titration analyses showed that residues of helix α1 (Q38, L39, and D40) and helix α4 (N68, T70, A71, V73, D74, I76, S78, and E82) of S4WIYLD and residues between the first and second β-strands (T9 and G10) and on β-strands 3 (R42, G47, K48, and Q49) and 4 (H68, R72, and L73) undergo significant chemical shift changes when the two proteins interact. A model of the complex, generated using HADDOCK, suggests that the N-terminal and C-terminal parts of S4WIYLD constitute a surface that interacts with charged residues close to the hydrophobic patch of ubiquitin. The WIYLD domains of the closely related SUVR1 and SUVR2 Arabidopsis proteins also bind ubiquitin, indicating that this is a general feature of this domain. The question of whether SUVR proteins act as both readers of monoubiquitinated H2B and writers of histone PTMs is discussed.