The two-source hypothesis (or 2SH) is an explanation for the synoptic problem, the pattern of similarities and differences between the three Gospels of Matthew, Mark, and Luke. It posits that the Gospel of Matthew and the Gospel of Luke were based on the Gospel of Mark and a hypothetical sayings collection from the Christian oral tradition called Q.

The two-source hypothesis emerged in the 19th century. B. H. Streeter definitively stated the case in 1924, adding that two other sources, referred to as M and L, lie behind the material in Matthew and Luke respectively. The strengths of the hypothesis are its explanatory power regarding the shared and non-shared material in the three gospels; its weaknesses lie in the exceptions to those patterns, and in the hypothetical nature of its proposed collection of Jesus-sayings. Later scholars have advanced numerous elaborations and variations on the basic hypothesis, and even completely alternative hypotheses. Nevertheless, "the 2SH commands the support of most biblical critics from all continents and denominations."

When Streeter's two additional sources, M and L, are taken into account, this hypothesis is sometimes referred to as the four-document hypothesis.