AIMS: To compare the outcomes of partners who participated in a telephone couples behavioural intervention to improve glycaemic control in persons with Type 2 diabetes with those of untreated partners of participants in an individual intervention or education; to explore 'ripple effects', i.e. positive behaviour changes seen in untreated partners. METHODS: The Diabetes Support Project was a three-arm randomized telephone intervention trial comparing outcomes of couples calls (CC), individual calls (IC) and diabetes education calls (DE). Couples included one partner with Type 2 diabetes and HbA ≥ 58 mmol/mol (7.5%). All arms received self-management education (two calls). CC and IC arms participated in 10 additional behaviour change calls. CC included partners, emphasizing partner communication, collaboration and support. Blinded assessments were performed at 4, 8 and 12 months. Partner outcomes were psychosocial (diabetes distress, relationship satisfaction, depressive symptoms), medical (BMI, blood pressure) and behavioural (fat intake, activity). RESULTS: Partners' (N = 268) mean age was 55.8 years, 64.6% were female and 29.9% were from minority ethnic groups. CC (vs. IC and DE) partners had greater reductions in diabetes distress, greater increases in marital satisfaction (4 and 8 months), and some improvements in diastolic BP. There were no consistent differences among arms in other outcomes. There was no evidence of a dietary or activity behaviour ripple effect on untreated partners, i.e. comparing partners in the IC and DE arms. CONCLUSIONS: A collaborative couples intervention resulted in significant improvements in partner diabetes distress and relationship satisfaction. There were no consistent effects on behavioural or medical partner outcomes, and no evidence of diet or activity behaviour ripple effects, suggesting that partners should be targeted directly to achieve these changes. (Clinical Trial Registry No: NCT01017523).