Chronic pain is a commonly occurring complication of cancer and can have negatively impact day-to-day functioning and well-being. The recommended treatment for all forms of chronic pain is cognitive behavioral therapy (CBT). However, access to this treatment in primary and specialist care is limited, including access to specialist pain clinics that do not routinely accept people with cancer-related pain. In addition, we know from clinical trials of pain-focused CBT that the symptom presentation of patients can vary greatly and may require interventions that are more tailored to their specific needs. The primary objective of the research is to test the feasibility, acceptability and preliminary efficacy of an internet-delivered, therapist-assisted, intervention for chronic pain with adults suffering from chronic cancer-related pain. The intervention is a form of CBT called Acceptance Commitment Therapy (ACT) and is based on the psychological flexibility model of pain. Psychological flexibility refers to an overall pattern of persistent behaviors characterized by a present-moment focus, acceptance rather than avoidance of distressing symptoms, and engagement with one's core values and goals. The intervention aims to improve overall functioning and well-being by helping participants to develop persistent patterns of behavior that align with with their core values and goals (e.g., personal, family, social, health, occupational) despite the presence of pain and other distressing symptoms. To achieve this aim, an existing pain-focused, internet-delivered, psychological flexibility based intervention was modified specifically for cancer-related pain with input from cancer patients and survivors. The 8-session, online intervention involves a mixture of written, audio, and videotaped materials, and instructions for experiential exercises, all of which are designed to reduce unhelpful coping behaviors and to increase psychological flexibility. Participants receive written support (and ask questions) from a therapist via the online platform where the intervention is hosted. Where necessary, participants may also access a limited number of face-to-face meetings (online) with their therapist. During the treatment phase, information from daily diary recordings (via smartphone) are used to provide individualized support to participants in the form of text messages from a therapist via the online platform. Where necessary, a limited number of face-to-face sessions (online) with a therapist are scheduled.