Background Having trustworthy health recommendations accessible in one single location is highly demanded by guideline users. Since 2014, open-access guidelines from the Spanish Society of Medical Oncology (SEOM) are available to facilitate clinical practice providing a practical view of the most relevant considerations concerning several cancer-related scenarios. To date, no independent assessment of its quality has been conducted. Objective To critically assess the methodological quality of SEOM guidelines on cancer treatment. Methods We conducted a critical review of clinical practice guidelines (CPGs). We searched MEDLINE/PubMed, guidelines repositories, and other sources from 2014 onwards. Appraisal of Guidelines for Research & Evaluation II (AGREE II) and AGREE-REX tools were used to assess the quality of the guidelines. Results We included and assessed 33 guidelines. Most of them (84.8% of CPGs, n= 28) were rated as “high quality” according to pre-defined criteria. The highest median standardized scores (96.3) were observed in the domain “clarity of presentation”, whereas “applicability” was distinctively low (31.4), with only one of the guidelines scoring more than 60%. Regarding “rigour of development”, the median standardized score was 74.3, and 28 of the guidelines scored more than 60%. SEOM´s guidelines did not include in their formulation the views and preferences of the target population. Moreover, they did not specify the updating methods either. Conclusions The SEOM guidelines on cancer treatment have been developed with acceptable methodological rigour although they have some drawbacks that could be improved in the future, such as clinical applicability and items regarding patient views and preferences. Relevance and importance to patients Clinical guidelines are also available to cancer patients and contain specific recommendations that aim to optimize healthcare. A valid guideline has the potential to impact on healthcare outcomes but also to facilitate empowerment by patients.