This study aimed to investigate the effect of online problem posing on students' problem-solving ability in mathematics. The study followed a quasi-experimental research with post-test that only controlled group design. The population was all of the 11th grade students in science classes of SMA Negeri 4 Singaraja in the academic year 2017/2018. A total of 119 students were sampled using cluster random sampling technique that were divided into 39 in experimental class, 40 in comparative class and 40 conventional class. The instrument used in this study was a mathematics problem-solving ability essay test. The data were analyzed using One Way ANOVA and Scheffe test. The result showed that there was a difference in terms of problem-solving ability in mathematics among the students who used online problem posing, problem posing and conventional learning. The result of Scheffe test showed that students' problem-solving ability in mathematics that used online problem posing was better. The study revealed that online problem posing had a significant effect on students' problem-solving ability in mathematics, which are better than problem posing or conventional learning.