Purpose This study aims to assess whether cancer-specific knowledge (CSK) is associated with membership in a cancer peer support group (PSG) and other factors. Methods A cross-sectional study investigated the CSK of 1121 cancer patients of various entities across Germany. CSK was measured with the BCKS-14, a 14-item knowledge instrument which was previously participatory developed with patient representatives and oncologists. Associations between CSK and PSG membership, sociodemographic factors, internet use, and preferences in medical decision-making were analysed with t-tests and multiple linear regressions. Results The t-test showed a statistically significant difference in CSK between members and non-members of PSGs. Knowledge for PSG members was on average 0.97 points higher (p < 0.001) and varied between 2 and 14 points compared to 0–14 points for non-members. Regression analysis revealed age, gender, time since diagnosis, education, internet use, and PSG activity to be statistically significant predictors. Younger (β = − 0.15; p < 0.001), female (β = 0.10; p = 0.001), higher educated patients (β = 0.27; p < 0.001) with and a diagnosis longer ago (β = 0.10; p = 0.002) who use the internet frequently for information seeking (β = 0.20; p ≤ 0.001) and members of cancer PSGs (β = 0.18; p ≤ 0.001) showed a higher CSK. Conclusion Overall, CSK of the participants shows a high degree of variance. CSK should be promoted for all patients and especially for older, newly diagnosed patients with low educational levels and PSGs introduced early on as they contribute to improving CSK among other benefits.