An air pump capable of twist-locking engagement includes a body housing, a head housing, a leakproof ring, an elastic sleeve, a pushing member and an end cap. The body housing has a cam section formed as a tube having a quarter-round section. The head housing has an opening segment. The head housing and the body housing are rotatably coupled with the leakproof ring arranged therebetween. The elastic sleeve is located at the opening segment of the head housing. The pushing member is located at one end of the elastic sleeve and received in the cam section. The end cap is screwed to the opening segment for pressing against and positioning the elastic sleeve. By a twisting operation of the air pump, the cam section can be turned to engage or release an air valve of an article to be inflated.