Background: Prior studies suggest that decision support persons (DSPs) involvement in breast cancer promotes greater deliberation and decision quality. Despite having the highest level of involvement, Latinx DSPs report the lowest satisfaction with their involvement. The reasons for this remain unknown. We examined the treatment decision-making experiences of Latinx DSPs, their influence on treatment deliberation, subjective decision quality (SDQ), and treatment received. Methods: Women with newly diagnosed early-stage breast cancer as reported to the Surveillance, Epidemiology, and End Results (SEER) registries of Georgia and Los Angeles County in 2014-2015 were surveyed. Participants identified the DSPs who played a key role in treatment decisions, who were also surveyed. We examined: (1) bivariate associations of DSP characteristics (e.g type, age, race/ethnicity, education, acculturation level) with DSP-reported level of engagement (informed about decisions, involvement (extent and satisfaction), and aware of patient preferences), (2) DSP engagement with patient-reported SDQ and treatment deliberation using multivariable linear regression with standardized scales (3), and treatment received by DSPs preferred treatment. Results: 2502 patients (68%) and 1203 eligible DSPs (70%) responded, resulting in 1,173 dyads, 292 where the patient identified as Latina, and 881 as non-Latina. Among Latina dyads, 78%, 17%, and 5% DSPs identified as Latinx, White, and Asian/Black/Other, respectively. Latinx DSPs within Latina/Latinx dyads were younger, had lower educational attainment and acculturation when compared to other dyads. The proportion of married/partnered status was not different across dyads, but the key DSP for the Latina-Latinx dyads was more often a daughter (37%), over a husband/partner (21%), compared to the other dyads. Latinx DSPs reported being more informed (adjusted mean 4.26, p = 0.058) compared to the other dyads, and being more informed was positively associated with higher patient SDQ (adjusted mean difference 0.176, p = 0.034), despite no difference in treatment deliberation. Overall, Latinx DSP had a higher preference for mastectomy, especially with reconstruction when compared to non-Latinx (40% vs 28%). Overall, 27% of Latinas (vs 13% non-Latina) underwent lumpectomy despite their DSP’s preference for mastectomy. Conclusions: These findings reveal that the key DSP for many married Latinas is often a daughter over spouse/partner. Our results suggest that including daughter/DSPs in treatment decisions and tailoring strategies to meet their information needs may positively impact Latina SDQ. Potential areas of improvement include surgical options preferences, where notable discrepancy was seen between Latinas and non-Latinas. Awareness of these differences can minimize treatment regret, improve decision quality, and ultimately outcomes in Latinas.