Background: One of the main aims of rehabilitation is to improve participation. Patient-Reported Outcomes Measurement Information System (PROMIS®) item banks ‘Ability to Participate in Social roles and Activities, (PROMIS-APS) and ‘Satisfaction with Social Roles and Activities’ (PROMIS-SPS) are promising options to measure participation, but the literature on PROMIS measures of (satisfaction with) participation across diagnoses in rehabilitation is limited. Therefore, the objective of this study was to describe levels of and changes in participation, as assessed with the PROMIS-APS and the PROMIS-SPS short forms, of patients in outpatient rehabilitation. Methods: This study had quantitative, observational design with assessments at admission and discharge. Consecutive patients treated between April and August 2018 receiving outpatient multidisciplinary rehabilitation were the population of this study. The following diagnosis categories were included: brain injury (e.g. stroke), spinal cord and nerve injury, neuromuscular disorder (e.g. lateral sclerosis), amputation, musculoskeletal condition (e.g. osteoarthritis) or heart or lung disease (e.g. myocardial infarction, chronic obstructive pulmonary disease). The main patient-reported outcomes (PRO) of this study were the short form of the PROMIS-APS (8 items, Dutch general population reference score 50.6 [SD 9.5]), and PROMIS-SPS (8 items, Dutch general population reference score 47.5 [SD 8.3]. Results: Of the 1279 patients invited, 777 (61%) completed the online forms at admission. Of those, 329 patients were invited at discharge, with 209 (64%) completing the forms. The mean (SD) T-scores of the PROMIS-APS and PROMIS-SPS were lower at admission (42.7 [SD 7.4]; (41.4 [SD 7.7]) and discharge (43.6 [SD 7.2]; (43.7 [SD 7.8]) than the Dutch general population. The change scores of the PROMIS-APS and PROMIS-SPS were 1.2 (95% CI 0.4–1.9; p = 0.004; effect size 0.16), and 2.4 (95% CI 1.6–3.2; p < 0.0001; effect size 0.31), respectively. In all diagnostic subgroups with > 30 paired measurements statistically significant improvements of PROMIS-APS, PROMIS-SPS or both were seen. Conclusions: Patients undergoing outpatient rehabilitation had, both at admission and discharge, considerably lower PROMIS-APS and PROMIS-SPS T-scores short forms than the general Dutch population, and showed small T-score improvements at discharge.