We extend the theory of almost coherent modules that was introduced in Almost ring theory by Gabber and Ramero (2003). Then we globalize it by developing a new theory of almost coherent sheaves on schemes and on a class of “nice” formal schemes. We show that these sheaves satisfy many properties similar to usual coherent sheaves, i.e., the amost proper mapping theorem, the formal GAGA, etc. We also construct an almost version of the Grothendieck twisted image functor f^! and verify its properties. Lastly, we study sheaves of p-adic nearby cycles on admissible formal models of rigid-analytic varieties and show that these sheaves provide examples of almost coherent sheaves. This gives a new proof of the finiteness result for 'etale cohomology of proper rigid-analytic varieties obtained before in Scholze's work p-adic Hodge theory for rigid-analytic varieties (2013).