The purpose of the four studies described in this article was to develop and test a new measure of competitive sport participants' intrinsic motivation, extrinsic motivation, and amotivation (self-determination theory; Deci & Ryan, 1985). The items for the new measure, named the Behavioral Regulation in Sport Questionnaire (BRSQ), were constructed using interviews, expert review, and pilot testing. Analyses supported the internal consistency, test-retest reliability, and factorial validity of the BRSQ scores. Nomological validity evidence was also supportive, as BRSQ subscale scores were correlated in the expected pattern with scores derived from measures of motivational consequences. When directly compared with scores derived from the Sport Motivation Scale (SMS; Pelletier, Fortier, Vallerand, Tuson, & Blais, 1995) and a revised version of that questionnaire (SMS-6; Mallett, Kawabata, Newcombe, Otero-Forero, & Jackson, 2007), BRSQ scores demonstrated equal or superior reliability and factorial validity as well as better nomological validity.