Abstract We report on a possible cloud–cloud collision in the DR 21 region, which we found through molecular observations with the Nobeyama 45 m telescope. We mapped an area of ∼8′ × 12′ around the region with 20 molecular lines including the 12CO(J = 1–0) and 13CO(J = 1–0) emission lines, and 16 of them were significantly detected. Based on the 12CO and 13CO data, we found five distinct velocity components in the observed region, and we call the molecular gas associated with these components “−42,”“−22,” “−3,” “9,” and “17” km s−1 clouds, after their typical radial velocities. The −3 km s−1 cloud is the main filamentary cloud ($∼ 31000\, M_{⊙ }$) associated with young massive stars such as DR21 and DR21(OH), and the 9 km s−1 cloud is a smaller cloud ($∼ 3400\, M_{⊙ }$) which may be an extension of the W75 region in the north. The other clouds are much smaller. We found a clear anticorrelation in the distributions of the −3 and 9 km s−1 clouds, and detected faint 12CO emission which had intermediate velocities bridging the two clouds at their intersection. These facts strongly indicate that the two clouds are colliding against each other. In addition, we found that DR21 and DR21(OH) are located in the periphery of the densest part of the 9 km s−1 cloud, which is consistent with results of recent numerical simulations of cloud–cloud collisions. We therefore suggest that the −3 and 9 km s−1 clouds are colliding, and that the collision induced the massive star formation in the DR21 cloud. The interaction of the −3 and 9 km s−1 clouds was previously suggested by Dickel, Dickel, and Wilson (1978, ApJ, 223, 840), and our results strongly support their hypothesis of the interaction.