Investigations into mindfulness and well-being have increasingly turned toward social outcomes such as compassion. However, research is needed to understand how mindfulness relates to compassion in real-world contexts. Moreover, the mechanisms underlying this relationship remain insufficiently understood, with self-related processes—particularly self-absorption and interdependent self-construal—representing important candidates. To address these gaps, we conducted a preregistered, ten-day diary study with n = 230 adults (Mage= 40, SD = 12; 50% female, 49% male, and 1% N/A) in heterosexual romantic relationships (Mlength of relationship = 15 years, SD = 11 years; n = 114 dyads with both members contributing). Partner ratings of compassion were collected to capture how mindfulness relates to a person’s compassion as perceived by close others in everyday life. We found that distinct facets of mindfulness showed different associations with compassion across levels of analysis: within persons, daily state increases in mindful acceptance corresponded with higher compassion (B = 0.19, SE = 0.06, p = 0.002, 95% CI[0.07, 0.31]), whereas between persons, greater trait mindful attention predicted greater compassion (B = 0.52, SE = 0.17, p = 0.002, 95% CI[0.19, 0.85]). We also found support for self-absorption as a mediating mechanism, as greater mindfulness (state and trait) was associated with more compassion through lower self-absorption. Although we expected this indirect effect to be stronger among individuals higher in interdependent self-construal, this moderation was not supported. Together, these findings highlight the importance of distinguishing state and trait processes in understanding the link between mindfulness and compassion in real-world contexts, and identify self-absorption as a key mechanism. Future work can address limitations of this work by investigating its generalizability to other populations.