The invention is an adhesive composition comprising a modified polyolefin-based resin in an amount of 45 mass % or more based on a total solid content of the adhesive composition and satisfying the specific following equation (I): α1/β1≥0.20  (I) [α1 represents a 180° peel adhesion strength (N/25 mm) as measured using the following measurement sample (A) under an environment at 85° C. in accordance with JIS Z0237: 2009, and β1 represents a water vapor permeability (g·m−2·day−1) as measured using the following measurement sample (B) under a condition at 40° C. and a relative humidity of 90%].One aspect of the invention provides an adhesive composition capable of obtaining a sealant excellent in sealing performance, a sealing sheet having an adhesive layer made by using the adhesive composition, and a sealed body obtained by sealing a seal subject with the sealing sheet.