IMPORTANCE There is little evidence to guide management of depressive symptoms in older people. OBJECTIVE To evaluate whether a collaborative care intervention can reduce depressive symptoms and prevent more severe depression in older people. DESIGN, SETTING, AND PARTICIPANTS Randomized clinical trial conducted from May 24, 2011, to November 14, 2014, in 32 primary care centers in the United Kingdom among 705 participants aged 65 years or older with Diagnostic and Statistical Manual of Mental Disorders (Fourth Edition) subthreshold depression; participants were followed up for 12 months. INTERVENTIONS Collaborative care (n=344) was coordinated by a case manager who assessed functional impairments relating to mood symptoms. Participants were offered behavioral activation and completed an average of 6 weekly sessions. The control group received usual primary care (n=361). MAIN OUTCOMES AND MEASURES The primary outcome was self-reported depression severity at 4-month follow-up on the 9-item Patient Health Questionnaire (PHQ-9; score range, 0-27). Included among 10 prespecified secondary outcomes were the PHQ-9 score at 12-month follow-up and the proportion meeting criteria for depressive disorder (PHQ-9 score 10) at 4- and 12-month follow-up. RESULTS The 705 participants were 58% female with a mean age of 77 (SD, 7.1) years. Four-month retention was 83%, with higher loss to follow-up in collaborative care (82/344 [24%]) vs usual care (37/361 [10%]). Collaborative care resulted in lower PHQ-9 scores vs usual care at 4-month follow-up. The proportions of participants meeting criteria for depression at 4-month follow-up were 17.2% (45/262) vs 23.5% (76/324), respectively (difference, −6.3% [95% CI, −12.8% to 0.2%]; relative risk, 0.83 [95% CI, 0.61-1.27]; P = .25) and at 12-month follow-up were 15.7% (37/235) vs 27.8% (79/284) (difference, −12.1% [95% CI, −19.1% to −5.1%]; relative risk, 0.65 [95% CI, 0.46-0.91]; P = .01). CONCLUSIONS AND RELEVANCE Among older adults with subthreshold depression, collaborative care compared with usual care resulted in a statistically significant difference in depressive symptoms at 4-month follow-up, of uncertain clinical importance. Although differences persisted through 12 months, findings are limited by attrition, and further research is needed to assess longer-term efficacy. TRIAL REGISTRATION isrctn.org Identifier: ISRCTN02202951