Although several studies have shown that dolphin-assisted therapy (DAT) may be beneficial for children with autism spectrum disorder (ASD), most of these studies have methodological weaknesses. This study tested the effectiveness of DAT in improving the social and communicative skills of children with ASD. We used a pretest- posttest design with a control group. The 48 participants were selected according to the following inclusion criteria: age between 4 and 5 years, diagnosis of ASD, and clear impairment in communication. Participants were randomly assigned to the groups: DAT and Therapy Without Dolphins (TWD). The research lasted 10 weeks. The intervention phase lasted 6 weeks and consisted of three weekly 45-min sessions, with a total of 18 sessions per participant. Therapy sessions for both groups, DAT and TWD, were conducted by a dolphin trainer and a health professional in the same facilities and under the same conditions, except for the use of the interaction with the dolphin as support for the activities. Three instruments were used to assess communicative and social skills: The Autism Diagnostic Observation Schedule-Generic, the Reynell Developmental Language scales, and the Vineland Adaptive Behavior scales. Significant improvements were observed between the pretests and posttests in both groups, DAT and TWD, on the total scores of all the scales, except for the Social Interaction Domain of the ADOS-G. These improvements were significantly greater for the DAT group on only two items of the Language and Communication Domain, ADOS-G: "frequency of vocalizations toward others" and "gestures" (η2 = 0.11, p < 0.05) in both cases. In conclusion, we found some evidence that DAT is a useful therapeutic intervention to foster the social and communication skills of children with ASD and that dolphins specifically can enhance the improvement of some communication-related aspects.