Understanding the fundamental laws of the universe is one of the main goals in theoretical physics. To unravel the mysteries of the universe, it is extremely important to understand its early stage. String theory is a candidate for a theory that describes the early universe. It is also believed to be a consistent theory of quantum gravity. Recent advances in string theory and holography have provided a lot of new perspectives in quantum gravity. Recently, there have been significant developments in modification or generalization of the Einstein's general relativity, which may be used in solving some of the problems in cosmology as well as giving us some hints to construct a fundamental theory of gravity. A detailed comparisons with recent cosmological observations will be important guidance towards our goal. The aim of this workshop is to bring together the experts of string theory, gravity and cosmology, and discuss recent progress in the research of string theory, supergravity, holography, quantum gravity, modified gravity, inflation, cosmology, and related topics. This workshop will be held as a joint activity with Korea Institute for Advanced Study (KIAS) and Yukawa Institute for Theoretical Physics, Kyoto University (YITP). This is the 7th KIAS-YITP joint workshop and you can find the links to the past KIAS-YITP workshops: 2005, 2007, 2009, 2011, 2013, 2015. This workshop is also supported in part by the JSPS Grant-In-Aid for Scientific Research (A) No. 16H02182.