This study aimed to examine the effect of problem-based learning (PBL) in developing science process skills (SPS) and learning achievement on the topic of 'Safety in Our Environment' in secondary schools in Tanzania. The study employed a quasi-experimental design. The participants for the study were Form One 'Stream E' students in the experimental school and Form One 'Stream D' students in the control school. Data were collected using a structured questionnaire and an achievement test. The results showed that the use of the PBL approach increased the mean scores from pre-test to post-test that were statistically significant at p = 0.00 in developing SPS compared to traditional teaching methods. Also, the use of PBL showed an increase in students' scores on the achievement test compared to the traditional teaching methods. Traditional teaching methods have the ability to retain achievement at a low level of cognitive ability as compared to PBL. Furthermore, PBL showed that there was no statistically significant difference in achievement based on gender as compared to the traditional teaching methods. The study recommends a continuous use of learner-centered approaches such as PBL in teaching and learning science subjects including Biology.