The invention provides a modified liquid diene polymer that allows a rubber composition to give a crosslinked product which contains a filler in a dispersed state ideal for attaining enhanced properties and which also exhibits excellent properties such as wet grip and abrasion resistance. The invention also provides a rubber composition including the modified liquid diene polymer, a crosslinked product thereof, and a tire which includes a portion including the composition or the crosslinked product. The modified liquid diene polymer is a product of modification, with a specific silane compound, of a liquid diene block copolymer including a polymer block (b′1) or (b″1) containing butadiene units and a polymer block (b′2) or (b″2). The modified liquid diene polymer has a functional group derived from the silane compound, and satisfies specific conditions.