INTRODUCTION: This study was a preliminary evaluation of a manualized, brief mindfulness-based intervention (MB-SI) for veterans with suicidal ideation (SI), admitted into an inpatient psychiatric unit (IPU). MATERIALS AND METHODS: A randomized, controlled pilot study of 20 veterans aged 18-70 years with SI, admitted into a psychiatric unit, assigned to treatment as usual (TAU) or MB-SI groups. Outcome data were collected at three time points: preintervention (beginning of first session), postintervention (end of last session), and 1-month postintervention. Primary outcomes were safety and feasibility. Secondary outcome measures were SI and behavior, mindfulness state and trait, cognitive reappraisal, and emotion regulation. Additionally, psychiatric and emergency department admissions were examined. Data analysis included Generalized Linear Models, Wilcoxon Signed-Rank, Mann-Whitney U, and Fisher's exact tests for secondary outcomes. RESULTS: Mindfulness-based intervention for suicidal ideation was feasible to implement on an IPU, and there were no associated adverse effects. Mindfulness-based intervention for suicidal ideation participants experienced statistically significant increase in Toronto Mindfulness Scale curiosity scores 1-month postintervention compared to preintervention and greater Toronto Mindfulness Scale decentering scores 1-month postintervention compared to TAU. Emotion Regulation Questionnaire Reappraisal scores significantly increased for the MB-SI group and significantly decreased for TAU over time. IPU and emergency department admissions were not statistically different between groups or over time. Both TAU and MB-SI participants experienced a significant reduction in Columbia-Suicide Severity Rating Scale-SI scores after the intervention. MB-SI participants experienced a higher increase in Five-Facet Mindfulness Questionnaire scores postintervention compared to TAU. CONCLUSIONS: Mindfulness-based intervention for suicidal ideation is feasible and safe to implement among veterans during an inpatient psychiatric admission with SI, as it is not associated with increased SI or adverse effects. Preliminary evidence suggests that MB-SI increases veterans' propensity to view experiences with curiosity while disengaging from experience without emotional overreaction. Further, more rigorous research is warranted to determine efficacy of MB-SI. TRIAL REGISTRATION: The clinicaltrials.gov registration number is NCT04099173 and dates are July 16, 2019 (initial release) and February 24, 2022 (most recent update).