This study aimed to examine the effect of informal learning environments on students' academic achievement and attitude levels in the Religious Culture and Moral Knowledge (RCMK) course. The study used a pretest-posttest experimental control group, quasi-experimental design model. An academic achievement test and Religion Culture and Moral Knowledge Course Attitude Scale were used as measurement tools. The study group comprised 116 students who continued their education in a private school at 4th and 5th grades. For the comparisons of attitude scale and achievement scores, independent samples t-tests were used. Before the posttests, the experimental group students attended mosque trips organized for the "Let's Get to Know the Mosque" unit. As a result of the study, it was observed that the attitude and academic achievement levels of the experimental group students towards the RCMK course increased after the implementation. It was found that informal learning had a significant and positive-positive effect on students' attitudes towards the course, and this effect was mostly on trust, benefit, interest, and liking sub-dimensions of the attitude scale, respectively.