To estimate excess health care costs in the 12 months following COVID-19 diagnosis. Retrospective cohort study using Blue Cross Blue Shield of Rhode Island claims incurred from January 1, 2019, to March 31, 2022, among commercial and Medicare Advantage members. Difference-in-differences analyses were used to compare changes in health care spend between the 12 months before (baseline period) and the 12 months after (post period) COVID-19 diagnosis for COVID-19 cases and contemporaneous matched controls without COVID-19. Overall, there were 7224 commercial and 1630 Medicare Advantage members with a COVID-19 diagnosis on/before March 31, 2021, each with a matched control, yielding a sample of 14,448 commercial and 3260 Medicare Advantage members. Among commercial members, 51.9% were aged 25 to 54 years and 54.0% were female. Among Medicare Advantage members, 94.2% were 65 years or older and 62.0% were female. Among commercial members, from the baseline period to the post period, total health care spend increased $41.61 (7.7%) per member per month (PMPM) more among COVID-19 cases compared with their matched controls. Among Medicare Advantage members, the difference-in-differences was greater, with spend increasing $97.30 (13.1%) PMPM more among cases compared with controls. The difference-in-differences was greatest for outpatient and professional services (both populations) and prescription services (Medicare Advantage only). COVID-19 diagnosis was associated with excess health care spend PMPM over the subsequent 12 months, highlighting the importance of societal preparations to support individuals' long-term health care needs following COVID-19 and as a part of future pandemic preparedness.