This study was conducted to examine the effects of training in Information Communication Technology-assisted (ICT-assisted) English language teaching on secondary school English language teachers' pedagogical knowledge and English language proficiency. To achieve this goal, a one-group within-subject quasi-experimental design was utilized. Twenty-two (n = 22) teachers selected from three government secondary schools in Arba Minch and Sawla towns of Gamo and Gofa zones of South Nations, Nationalities, and the Peoples Regional State of Ethiopia were included in the study using a comprehensive sampling technique. Four rounds of repeated measurements were used to collect data and Friedman's test was used to compare the mean scores to examine the effects of the training on the teachers' pedagogical knowledge and English language proficiency. Related-samples Kendall's coefficient of concordance (effect size) was also run using the same non-parametric test of Friedman's test to exmine the magnitude of the effect. The results revealed that training in ICT-assisted teaching has a significant effect on the pedagogical knowledge and English language proficiency of the teachers. Moreover, the independent variable (training in ICT-assisted English language teaching) has a strong effect (effect size) on their pedagogical knowledge and English language proficiency. Based on the results, in-service teacher training in ICT-assisted teaching is recommended to harness the potential of new technologies for English language teachers' professional development.