Abstract: English Language speaking practices in Kenyan secondary schools are insufficient to improve students’ progress in speaking skills. The purpose of the study was to investigate the effect of interpersonal and social skills in Cooperative Learning (CL) on secondary school learners’ English-speaking skills in Nandi Central Sub-County. The study used the quasi experimental with a non-equivalent control group pre-test post-test design under the mixed methods approach. The sample size was all the form three learners selected schools. The instruments of the study included learners’ questionnaire, Learners’ English-Speaking Skills Achievement Test and observation guide. These instruments were validated and a Pilot test was done prior to the study whereby a Cronbach coefficient of 0.78 was computed. Quantitative data was analyzed using descriptive and inferential statistics and presented in tabular form, while analysis from the observation guide was used for triangulation. The study revealed that there is a statistically significant effect of interpersonal and social skills (t=2.953; df=66), on learners’ achievement in English language speaking. It was concluded that, interpersonal and social skills positively affect learners’ achievement in English language speaking. The findings of this study are beneficial to the learners of English language and their teachers in the teaching and learning of speaking skills. They are also important to teacher educators, curriculum developers and the ministry of Education on decision making. The researcher recommends that teachers of English should consider incorporating Cooperative Learning in their teaching of English language speaking skills.