Introduction: Comprehensive geriatric assessment (CGA) has been shown to reduce frailty in older patients in general. In older patients with cancer, frailty affects quality of life (QoL), physical function, and survival. However, few studies have examined the effect of CGA as an additional intervention to antineoplastic treatment. This protocol presents a randomized controlled trial, which aims to evaluate the effects of CGA-based interventions in older patients with cancer and Geriatric 8 (G8) identified frailty. Materials and Methods: This randomized controlled trial will include patients, age 70+ years, with solid malignancies and G8 frailty (G8 ≤ 14). Patients will be separated into two groups, with different primary endpoints, depending on palliative or curative antineoplastic treatment initiation, and subsequently randomized 1:1 to either CGA with corresponding interventions or standard of care, along with standardized antineoplastic treatment. A geriatrician led CGA with corresponding interventions and clinical follow-up will be conducted within one month of antineoplastic treatment initiation. The interdisciplinary CGA will cover multiple geriatric domains and employ a standard set of validated assessment tools. Primary endpoints will be physical decline measured with the 30-s Chair-Stand-Test at three months (palliative setting) and unplanned hospital admissions at six months (curative setting). Additional outcomes include QoL, treatment toxicity and adherence, occurrence of polypharmacy, potential drug interactions, potential inappropriate medications, and survival. The primary outcomes will be analyzed using a mixed model regression analysis (30-s chair stand test) and linear regression models (unplanned hospitalizations), with an intention to treat approach. Power calculations reveal the need to enroll 134 (palliative) and 188 (curative) patients. Discussion: The present study will examine whether CGA, as an additional intervention to antineoplastic treatment, can improve endpoints valued by older patients with cancer. Inclusion began November 2020 and is ongoing, with 37 and 29 patients recruited April 15th, 2021. Registration: NCT04686851