Background Management of symptoms is essential in reducing the symptom burden of cancer patients. The effect of symptom diaries on symptom management to date has been evaluated only in ambulatory settings. Objective The aims of this study were to identify the key facilitators for successful implementation of symptom diaries on oncology wards from patients’ and professionals’ perspectives and to evaluate implementation outcomes. Methods In 2 cycles of action research, the Utrecht Symptom Diary (USD) was implemented on 3 oncology wards and a daycare unit. Key facilitators for implementation were identified by thematic coding of interviews. The effect of the implementation was evaluated in cycle II in a pretest-posttest design. We performed statistical tests (Mann-Whitney/t test/χ 2) on Symptom Management Performance items in questionnaires and medical records. Results We interviewed 25 patients, 8 doctors, and 25 nurses. Seven key facilitators for implementation emerged. After implementation of the USD in cycle II, Symptom Management Performance was significantly (P < .05) improved for patients (3/12 items, n = 33 pretest/26 posttest) and professionals (6/12 items, n = 21 pretest/19 posttest). Significantly more symptoms (P = .00), working hypotheses (P = .023), treatment plans (P = .00), and interventions (P = .00) were reported (n = 47 pretest/47 posttest). Conclusions Implementation of the USD significantly improved symptom management in oncology wards. We recommend (1) using a diagnosis-specific diary; (2) making clear, individualized working-arrangements; (3) training professionals; (4) using the plan-do-check-act cycle; (5) acting multidisciplinary; (6) providing guidelines and training; and (7) assuring adequate information communications technology (ICT). Implications Symptom diaries are increasingly used, but implementation is challenging. This study provides knowledge on their benefits and an evidence-based strategy for implementation with positive outcomes achieved in patient care.