For patients with chronic pain conditions such as rheumatoid arthritis (RA), who experience elevated levels of distress, tailored-guided internet-based cognitive-behavioral treatment may be effective in improving psychological and physical functioning, and reducing the impact of RA on daily life. A multicenter, randomized controlled trial was conducted for RA patients with elevated levels of distress as assessed by a disease-specific measure. The control group (n = 71) received standard care and the intervention group (n = 62) additionally received an internet-based tailored cognitive-behavioral intervention. Main analyses were performed using a linear mixed model estimating differences between the intervention and control groups in scores of psychological functioning, physical functioning, and impact of RA on daily life at preassesment and postassessment, and at 3, 6, 9, and 12 months. Patients who received the internet-based intervention reported a larger improvement in psychological functioning compared with the control group, indicating less depressed mood (P < 0.001, d = 0.54), negative mood (P = 0.01, d = 0.38), and anxiety (P < 0.001, d = 0.48) during the course of the 1-year follow-up period. Regarding physical functioning, a trend was found for the intervention group reporting less fatigue than the control group (P = 0.06, d = 0.24), whereas no effect was found on pain. No effects were found for the impact of RA on daily life, except for the intervention group experiencing fewer role limitations due to emotional problems (P < 0.001, d = 0.53). Offering guided internet-based cognitive-behavioral therapy is a promising development to aid patients with psychological distress particularly in improving psychological functioning. Further research on adherence and specific intervention ingredients is warranted.