Background
Loneliness is a global issue, and primary care physicians play an important role in assessing and intervening with loneliness. This study aimed to examine the association between having a usual source of care (USC) or a good quality of primary care, and loneliness.

Methods
This cross-sectional study was conducted in Japan in 2022. A total of 6,000 residents were randomly sampled from the general population, aged 20–74 years. The outcome was the total score of the University of California, Los Angeles (UCLA) 3-item loneliness scale. The exposure included USC and the Person-Centered Primary Care Measure (PCPCM), which assesses the quality of primary care. We conducted a linear regression analysis to adjust for age, sex, educational status, annual household income, self-rated health, living status (whether alone or not), and the existence of physical health problems.

Results
Of the 6,000 residents, 1,277 responded to the survey. The median score of the UCLA 3-item loneliness scale was 6.0 and the mean total score of the PCPCM was 2.62. Of the 1,277 individuals, 713 (55.8%) had USC. Having USC was significantly associated with lower scores on the UCLA 3-item loneliness scale; the coefficient was −0.34 (95% confidence interval (CI): −0.57 to −0.12). Also, the total PCPCM score was significantly associated with lower loneliness scores; the coefficient was −0.56 (P < 0.001, 95% CI: −0.78 to −0.35).

Conclusions
Having USC and a better quality primary care were associated with a lower loneliness score. The quality of primary care could be a factor to mitigate patient loneliness.