Background: There is evidence showing that fundamental movement skills and physical activity are related with each other. The ability to perform a variety of fundamental movement skills increases the likelihood of children participating in different physical activities throughout their lives. However, no fundamental movement skill interventions implemented with junior high school students have yet been reported. Purpose: To investigate the changes in students' locomotor, manipulative, and balance skills and their level of self-reported physical activity during the specific intervention program aiming to increase students' fundamental movement skills in Finnish junior high school physical education. Participants and setting: 446 Finnish Grade 7 students ([approximately] 13 years old) from Central Finland. Research design: A quasi-experimental intervention study with pre-, middle-, post-, and retention tests was used. The experimental group consisted of 199 students taught by 4 teachers and the control group included 247 students taught by 6 teachers. The intervention consisted of 33 sessions, 25 minutes of each, and included training of fundamental movement skills within naturalistic physical education classes during one academic year. Data collection: There were four waves of measurement for fundamental movement skill tests and self-reports of physical activity. Data analysis: Repeated measures MANOVAs were conducted to analyze interactions between condition (experimental/control) and time (four measurement points). In cases with an interaction between condition and time, follow-up post hoc tests were performed to examine which group means differed from each other. Findings: Results indicated significant condition x time interaction in static balance, dynamic balance, balance skills sum score, movement skills sum score, and self-reported physical activity. The experimental group demonstrated more positive development of these variables compared to the control group. Conclusions: This study revealed that it is possible to develop junior high school students' fundamental skills through physical education. These changes seem to be more obvious when focusing on students' balance skills. Although further longitudinal investigation is needed, the fundamental movement skill intervention seemed also to prevent the typical decline in physical activity within junior high school students. (Contains 2 tables.)