Patients with cancer require adequate preparation in self-management of treatment toxicities to reduce morbidity that can be achieved through well-designed digital technologies that are developed in co-design with patients and end users. We undertook a user-centered co-design process in partnership with patients and other knowledge end users to develop and iteratively test an evidence-based and theoretically informed web-based cancer self-management program (I-Can Manage). The specific study aims addressed in 2 phases were to (1) identify from the perspective of patients with cancer and clinicians the desired content, features, and functionalities for an online self-management education and support (SMES) program to enable patient self-management of treatment toxicities (phase 1); (2) develop the SMES prototype based on human-centered, health literate design principles and co-design processes; and (3) evaluate usability of the I-Can Manage prototype through user-centered testing (phase 2). We developed the I-Can Manage program using multiperspective data sources and based on humanistic and co-design principles with end users engaged through 5 phases of development. We recruited adult patients with lung, colorectal, and lymphoma cancer receiving systemic treatments from ambulatory clinics in 2 regional cancer programs for the qualitative inquiry phase. The design of the program was informed by data from qualitative interviews and focus groups, persona and journey mapping, theoretical underpinnings of social cognitive learning theory, and formalized usability testing using a cognitive think-aloud process and user satisfaction survey. A co-design team comprising key stakeholders (human design experts, patients/caregiver, clinicians, knowledge end users, and e-learning and digital design experts) was involved in the developmental process. We used a cognitive think-aloud process to test usability and participants completed the Post-Study System Usability Questionnaire (PSSUQ). In the initial qualitative inquiry phase, 16 patients participated in interviews and 19 clinicians participated in interviews or focus groups and 12 key stakeholders participated in a persona journey mapping workshop to inform development of the program prototype. The I-Can Manage program integrates evidence-based information and strategies for the self-management of treatment toxicities and health-promoting behaviors in 6 e-learning modules (lay termed "chapters"), starting with an orientation to self-management. Behavioral exercises, patient written and video stories, downloadable learning resources, and online completion of goals and action plans were integrated across chapters. Patient participants (n=5) with different cancers, gender, and age worked through the program in the human factors laboratory using a cognitive think-aloud process and all key stakeholders reviewed each chapter of the program and approved revisions. Results of the PSSUQ (mean total score: 3.75) completed following the cognitive think-aloud process (n=5) suggest patient satisfaction with the usability of I-Can Manage. The I-Can Manage program has the potential for activating patients in self-management of cancer and treatment toxicities but requires testing in a larger randomized controlled trial.