In this study, the effects of number sense-based instruction on students’ mathematics-related self-efficacy and performance were studied. Self-efficacy relates to both mathematics and number sense, while student performance consists of their number sense, problem solving, general mathematical achievement, and recognition of mathematics in daily life. This was an experimental study where both quantitative research and pretest/posttest control group designs were utilized. The fifteen-week study was conducted with a group of sixth-grade students attending a public school located in the Cankaya district in Ankara Province, Turkey, including thirteen weeks to implement the instructional plan and two weeks for employing the scales. During this process, while students in the test group were introduced to number sense-based training, the control group adhered to the regular syllabus where teachers based most of their instruction on textbooks. The study, beyond its demonstration of number sense, also evaluates number sense development in the context of an instructional plan designed for long-term implementation and contributes to the study of changes in other variables involved in development. Throughout the study, the Number Sense Test was used to identify the number sense of students. The Mathematics Self-Efficacy Perception Scale was used to assess their self-efficacy. The Number Sense Self-Efficacy Scale was used to identify their number sense self-efficacy. The Mathematics in Daily Life Survey was used to assess their recognition of mathematics in daily life. The Problem-Solving Test was used to assess their achievement in solving problems, and end-of-term grading was used to identify their overall mathematical achievement. Data were analyzed with multivariate analysis of variance (MANOVA) tests using SPSS 15.0 (SPSS Inc., Chicago, Il). The results of the analyses revealed that students who received instruction based on number sense experienced significant improvement in their number sense, their recognition of mathematics in daily life, and their problem-solving achievements compared to students who received only regular instruction. Number sense-based instruction did not create any significant changes in mathematics self-efficacy, number sense self-efficacy, or the overall mathematical achievements of the students.