Purpose The known advantage of exercise for older adults who had knee pain is limited by low adherence to an exercise program. This study aims to determine the effect of the LINE application on action and coping plans on exercise adherence, self-efficacy for exercise, specific self-efficacy (task, maintenance and recovery), functional performance (knee range of motion, time up and go, 30-s chair stand and knee outcome for activities of daily living) and knee pain scale among older adults with knee pain in suburban Bangkok, Thailand. Design/methodology/approach A total of 86 participants aged between 50 and 65 years old were divided into two groups for a period of 14 weeks: intervention (received action and coping plans via the LINE application) and control group (received usual care). All outcomes were measured at baseline and posttest except exercise adherence, which was collected postintervention. Findings A significant difference between intervention and control groups across all outcomes. Within group comparisons before and after, the intervention indicates that participants of the intervention group significantly improved posttest. In conclusion, using mobile health technology in combination with action and coping plans was found to enhance older adults’ exercise adherence and motivation, thus, decreasing knee pain while increasing functionality. Originality/value This study provides new insight into the combination of action and coping plans implementing with instant messaging through the LINE application which had a positive impact on enhance exercise adherence and knee functional performance of Thai elderly with knee pain.