Background People with aphasia may improve their communication with speech and language therapy many months/years after stroke. However, NHS speech and language therapy reduces in availability over time post stroke. Objective This trial evaluated the clinical effectiveness and cost-effectiveness of self-managed computerised speech and language therapy to provide additional therapy. Design A pragmatic, superiority, single-blind, parallel-group, individually randomised (stratified block randomisation, stratified by word-finding severity and site) adjunct trial. Setting Twenty-one UK NHS speech and language therapy departments. Participants People with post-stroke aphasia (diagnosed by a speech and language therapist) with long-standing (> 4 months) word-finding difficulties. Interventions The groups were (1) usual care; (2) daily self-managed computerised word-finding therapy tailored by speech and language therapists and supported by volunteers/speech and language therapy assistants for 6 months plus usual care (computerised speech and language therapy); and (3) activity/attention control (completion of puzzles and receipt of telephone calls from a researcher for 6 months) plus usual care. Main outcome measures Co-primary outcomes – change in ability to find treated words of personal relevance in a bespoke naming test (impairment) and change in functional communication in conversation rated on the activity scale of the Therapy Outcome Measures (activity) 6 months after randomisation. A key secondary outcome was participant-rated perception of communication and quality of life using the Communication Outcomes After Stroke questionnaire at 6 months. Outcomes were assessed by speech and language therapists using standardised procedures. Cost-effectiveness was estimated using treatment costs and an accessible EuroQol-5 Dimensions, five-level version, measuring quality-adjusted life-years. Results A total of 818 patients were assessed for eligibility and 278 participants were randomised between October 2014 and August 2016. A total of 240 participants (86 usual care, 83 computerised speech and language therapy, 71 attention control) contributed to modified intention-to-treat analysis at 6 months. The mean improvements in word-finding were 1.1% (standard deviation 11.2%) for usual care, 16.4% (standard deviation 15.3%) for computerised speech and language therapy and 2.4% (standard deviation 8.8%) for attention control. Computerised speech and language therapy improved word-finding 16.2% more than usual care did (95% confidence interval 12.7% to 19.6%; p < 0.0001) and 14.4% more than attention control did (95% confidence interval 10.8% to 18.1%). Most of this effect was maintained at 12 months (n = 219); the mean differences in change in word-finding score were 12.7% (95% confidence interval 8.7% to 16.7%) higher in the computerised speech and language therapy group (n = 74) than in the usual-care group (n = 84) and 9.3% (95% confidence interval 4.8% to 13.7%) higher in the computerised speech and language therapy group than in the attention control group (n = 61). Computerised speech and language therapy did not show significant improvements on the Therapy Outcome Measures or Communication Outcomes After Stroke scale compared with usual care or attention control. Primary cost-effectiveness analysis estimated an incremental cost per participant of £732.73 (95% credible interval £674.23 to £798.05). The incremental quality-adjusted life-year gain was 0.017 for computerised speech and language therapy compared with usual care, but its direction was uncertain (95% credible interval –0.05 to 0.10), resulting in an incremental cost-effectiveness ratio of £42,686 per quality-adjusted life-year gained. For mild and moderate word-finding difficulty subgroups, incremental cost-effectiveness ratios were £22,371 and £28,898 per quality-adjusted life-year gained, respectively, for computerised speech and language therapy compared with usual care. Limitations This trial excluded non-English-language speakers, the accessible EuroQol-5 Dimensions, five-level version, was not validated and the measurement of attention control fidelity was limited. Conclusions Computerised speech and language therapy enabled additional self-managed speech and language therapy, contributing to significant improvement in finding personally relevant words (as specifically targeted by computerised speech and language therapy) long term post stroke. Gains did not lead to improvements in conversation or quality of life. Cost-effectiveness is uncertain owing to uncertainty around the quality-adjusted life-year gain, but computerised speech and language therapy may be more cost-effective for participants with mild and moderate word-finding difficulties. Exploring ways of helping people with aphasia to use new words in functional communication contexts is a priority. Trial registration Current Controlled Trials ISRCTN68798818. Funding This project was funded by the National Institute for Health Research (NIHR) Health Technology Assessment programme and will be published in full in Health Technology Assessment; Vol. 24, No. 19. See the NIHR Journals Library website for further project information. The Tavistock Trust for Aphasia provided additional support to enable people in the control groups to experience the intervention after the trial had ended.