A marine vessel steering system includes a basic target turning angle computing unit that computes a basic target turning angle δo* common to two outboard motors based on a steering angle θ detected by a steering angle sensor. A traveling state determining unit determines whether a traveling state of a marine vessel is a straight traveling state based on the received basic target turning angle δo*. When the traveling state of the marine vessel is determined as being the straight traveling state, a target turning angle computing unit determines, based on the basic target steering angle δo* and a straight traveling toe angle φs stored in a nonvolatile memory, target turning angles δ* of the two outboard motors such that a toe angle between the two outboard motors is equal to the straight traveling toe angle φs.