This study examined how social isolation and subjective loneliness impacted subjective well-being across Japan, with the aim of offering policy recommendations. We analyzed data (N = 5,234, age:15-89) obtained through a Japanese population-based national survey conducted in March 2021. First, we found that mean life satisfaction was statistically lowest among the subgroup of respondents who lived alone (approximately 20% of the total), with more pronounced impacts in males. We also found that the frequency of social networking site (SNS) usage was significantly associated with overall life satisfaction, exhibiting a J-curve (three to four times per week was better than everyday usage). The frequency of interaction with others, number of SNS friends, employment/wage satisfaction, family budget/asset satisfaction, subjective health satisfaction, and subjective connectedness were also related to well-being in a dose-dependent manner (Trend P<0.001). Second, our multiple logistic regression model showed that the above relationships were independently and statistically significant (p<0.05). Moreover, a stratified analysis showed that living alone significantly affected these relationships, which suggests that this condition should specifically be addressed in the context of efforts aimed at enhancing subjective well-being. In addition, respondents who lived alone had low subjective connectedness, which also independently influenced well-being, as shown by our multivariate logistic model. In sum, these findings indicate that national well-being enhancement policies should particularly consider social isolation and loneliness, including issues stemming from low social connectedness.