Background: Individuals with rectal cancer require a number of pre-treatment investigations to determine the local-regional and overall stage of disease. Stage of rectal cancer determines treatment plan; therefore incomplete or inadequate staging may result in sub-optimal care and outcomes. Methods: This is a population based study of all individuals undergoing surgical resection for rectal cancer in Ontario, Canada (population 14.6 million) between 2010 and 2019. Individuals were identified using the Ontario Cancer Registry which includes approximately 95% of all incident cases of rectal cancer in the province. “Complete Staging” in Rectal Cancer has previously been defined and includes assessments of distant metastasis, local-regional stage and an attempt at colonic assessment for synchronous lesions. Patient and care provider characteristics, staging investigations, stage of disease, treatments and long-term outcomes were determined using linked administrative databases. Results: The study cohort included 10,957 individuals with rectal cancer; 24% Stage I, 21% Stage II, 40% Stage III, 7% Stage IV, 8% Missing Stage. The average age was 65 (STD 12.6) and males accounted for 63% of the study population. Incomplete staging occurred in 26%, with incomplete local regional staging being the most common deficiency (21%). Increasing patient age (< 0.001), low volume surgeons (P < 0.001) and low volume hospitals (P < 0.001) were associated with incomplete staging. There was significant regional variation in the completeness of staging (low 68% - High 84%). In those with locally advanced rectal cancer (Stage II and Stage III), incomplete staging was associated with lower rates of preoperative radiation oncology assessments (27% vs. 80%, P < 0.001) and medical oncology assessments (12% vs. 39%, P < 0.001). In addition, incomplete staging was associated with lower rates of any radiation (pre or postoperative) (45% vs. 82%, P < 0.001), lower rates of preoperative neoadjuvant therapy (22% vs. 74%, P < 0.001) and higher rates of post operative radiation (23% vs. 8.3%, P < 0.001). Those with incomplete staging had a lower 5 year overall survival (73% vs. 81%, P < 0.001). Conclusions: In this study, we identified several modifiable risk factors for incomplete staging prior to treatment for rectal cancer. Incomplete staging likely results in suboptimal care in this population, as demonstrated by less oncology referrals and less use of appropriate neoadjuvant therapy.