Abstract. We equip the categorified quantum group attached KLR algebra and an arbitrary choice of scalars with duality functor which is cyclic, that is, such that f = f * * for all 2-morphisms f . This is accomplished via a modified diagrammatic formalism.Consider the 2-category U Q (g) (as defined in [3,5,13]) which categorifies a Kac-Moody Lie algebra g. In this 2-category, every 1-morphism possesses a left dual and a right dual. Standard arguments show that the left and right duals (which are isomorphic) are well-defined up to unique isomorphism; however they are not well-defined as 1-morphisms. That is, the operations of left and right dual are anafunctors, not functors. In particular, every 1-morphism is isomorphic to its double-dual, but there is no fixed isomorphism intrinsic to the 2-category structure.However, we can choose a right duality functor, which allows us to fix a left and right dual. In fact, this duality will be strict: any 1-morphism will be equal to its double dual. One such functor is supplied by the functor τ defined in [6, 3.46], that is by rightward rotation by 180 • . However, in the graphical calculus defined in [5], this functor lacks one of the basic properties we expect from a duality: while a 1-morphism is equal to its double dual, for a 2-morphism f : u → v, we will not necessarily have that f and f * * are equal.If the equality f = f * * does hold, we call the resulting duality cyclic. In this case, the functor of double dual is isomorphic to the identity; thus, the identity map defines a (strictly) pivotal structure on this 2-category. This property is also equivalent to the induced biadjunction on the objects (u, u * ) being cyclic. In diagrammatic terms, this means that a morphism is unchanged by a full 360 • rotation. It follows that the string diagram calculus which uses a cyclic duality functor enjoys a topological invariance that greatly simplifies computations. Furthermore, a pivotal structure is necessary for defining convolution in the Hochschild cohomology of a representation of this 2-category; in particular, it plays a key role in the authors' previous work on traces of categorified quantum groups, and related work of Shan, Varagnolo and Vasserot [1,14,17].Our work is motivated by work of Brundan [3], which shows that the 2-category introduced in [5] can be defined as in [13], where the functor F i is the right dual of E i (by definition), but there is no a priori connection between F i and the left dual of E i . In order to define a duality functor, we must thus choose an isomorphism of E i to the right dual of F i (that is, of F i with the left dual of E i ).In [3, 1.2], one such isomorphism is defined, which matches the choice implicit in [5]. Unfortunately, as [5, (2.5)] shows, this duality is usually not cyclic; in particular, it is not for a generic choice of parameters, or for the choice which is most important in geometric and representation theoretic applications, such as [4,15].In this paper, we introduce a 2-category U cyc Q (g) for an arbitrary choice of...