Positive psychological constructs are associated with superior outcomes in cardiac patients, but there has been minimal study of positive psychology (PP) interventions in this population. Our objective was to describe the intervention development and pilot testing of an 8-week phone-based PP intervention for patients following an acute coronary syndrome (ACS). Initial intervention development and single-arm proof-of-concept trial, plus comparison of the PP intervention to a subsequently-recruited treatment as usual (TAU) cohort. PP development utilized existing literature, expert input, and qualitative interview data in ACS patients. In the proof-of-concept trial, the primary outcomes were feasibility and acceptability, measured by rates of exercise completion and participant ratings of exercise ease/utility. Secondary outcomes were pre-post changes in psychological outcomes and TAU comparisons, measured using effect sizes (Cohen’s d). The PP intervention and treatment manual were successfully created. In the proof-of-concept trial, 17/23 PP participants (74 %) completed at least 5 of 8 exercises. Participants rated the ease (M = 7.4/10; SD = 2.1) and utility (M = 8.1/10, SD = 1.6) of PP exercises highly. There were moderate pre-post improvements (ds = .46–.69) in positive affect, anxiety, and depression, but minimal effects on dispositional optimism (d = .08). Compared to TAU participants (n = 22), PP participants demonstrated greater improvements in positive affect, anxiety, and depression (ds = . 47–.71), but not optimism. A PP intervention was feasible, well-accepted, and associated with improvements in most psychological measures among cardiac patients. These results provide support for a larger trial focusing on behavioral outcomes.