The sensation of being encouraged is closely linked to subjective well-being (SWB). This study examined whether sustained encouragement from a conversational robot could enhance users’ SWB. A preliminary survey with 48 participants identified two types of encouraging utterances with significantly different effects on participants’ perceived levels of encouragement. Based on this survey, we recruited 6 participants for a 15-day experiment with a conversational robot (2 participants completed only 9 days). They interacted with the robot every two days and were randomly assigned to either a high-efficacy or low-efficacy encouraging utterances group. SWB was assessed through questionnaires and daily diaries, while interviews explored whether observed changes were attributable to the robot interactions. Results suggest that high-efficacy encouraging utterances may positively influence users’ SWB. This study provides theoretical support for using conversational robots to enhance human SWB through sustained high-efficacy encouragement. Future research should explore these effects in diverse populations and over longer durations.