Mutants of the C4-symmetric protein RhuA were designed to self-assemble into two-dimensional crystalline lattices with precise spatial arrangements and patterns; the lattices of one of the variants are auxetic and deform perpendicularly to an applied force in a way that is contrary to what is generally expected in typical materials. Auxetic materials are those that, because of their internal structure, deform perpendicularly to an applied force in a manner opposite to what is generally expected. So, when stretched, they get thicker across their width, and when compressed they get thinner. Akif Tezcan and colleagues have created a crystalline protein lattice that demonstrates such behaviour, dependent on the positioning and type of linkages between each individual protein unit. They designed mutants of the C4-symmetric protein RhuA to self-assemble into two-dimensional crystalline lattices with precise spatial arrangements and patterns. Disulfide bonds and metal-mediated coordination between units provide a balance between robustness and flexibility, such that large, low-defect lattices are formed that exhibit coherent rotational motion in response to an applied stress. Two-dimensional (2D) crystalline materials possess unique structural, mechanical and electronic properties1,2 that make them highly attractive in many applications3,4,5. Although there have been advances in preparing 2D materials that consist of one or a few atomic or molecular layers6,7, bottom-up assembly of 2D crystalline materials remains a challenge and an active area of development8,9,10. More challenging is the design of dynamic 2D lattices that can undergo large-scale motions without loss of crystallinity. Dynamic behaviour in porous three-dimensional (3D) crystalline solids has been exploited for stimuli-responsive functions and adaptive behaviour11,12,13. As in such 3D materials, integrating flexibility and adaptiveness into crystalline 2D lattices would greatly broaden the functional scope of 2D materials. Here we report the self-assembly of unsupported, 2D protein lattices with precise spatial arrangements and patterns using a readily accessible design strategy. Three single- or double-point mutants of the C4-symmetric protein RhuA were designed to assemble via different modes of intermolecular interactions (single-disulfide, double-disulfide and metal-coordination) into crystalline 2D arrays. Owing to the flexibility of the single-disulfide interactions, the lattices of one of the variants (C98RhuA) are essentially defect-free and undergo substantial, but fully correlated, changes in molecular arrangement, yielding coherently dynamic 2D molecular lattices. C98RhuA lattices display a Poisson’s ratio of −1—the lowest thermodynamically possible value for an isotropic material—making them auxetic.