Background: Most adults with Down syndrome develop Alzheimer's disease (AD) pathology in their 30s, yet research into cognitive health programmes for this group remains limited. Method: A mixed-methods feasibility randomised control trial (RCT) evaluated an adapted, manualised group-based cognitive stimulation therapy (CST) programme for adults with Down syndrome (N = 12; M[subscript age] = 30) without dementia. Participants were randomly assigned to CST (n = 6) or control (services as usual; n = 6), with assessments at baseline, post-programme, and four-month follow-up by a blinded researcher. Results: The adapted CST was feasible, with high attendance, strong satisfaction, and good CST programme fidelity (all > 85%). CST participants showed significant gains in adaptive behaviour at post-programme, maintained at follow-up, and a trend towards improved episodic memory at post-programme. Conclusion: Manualised group-based CST can be successfully adapted for younger adults with Down syndrome and shows promise in supporting cognitive health for this population.