The study aimed to explore the impact of an SDM-based professional development program on teacher discourse. Two types of discourse, authoritative and dialogic discourses, was the focus of the search. From a Bakhtinian standpoint, authoritative words are viewed as located in a distanced zone, do not reflect any individual point of view, and are not disputable. Moreover, in these words, one hears only one, single voice. Whereas, internally persuasive (dialogic) words are freely developed, applicable to new material and conditions, and, in these words, one hears at least two voices. A total of seventeen teachers volunteered and participated in a four-week professional development program. The program provided information about and classroom uses of authoritative and dialogic talk. After the program had been fulfilled, the classroom activities were videotaped and then transcribed. The transcriptions were later analyzed deductively. The results indicated that the SDM-based program had a positive and statistically significant impact on teacher dialogic discourse. The analysis revealed that after the program, all the teachers altered their discourse to a more dialogic one. It was specifically observed that a total of eleven teachers' dialogic-authoritative talk ratio was over 7/3.