Background and objectives Diabetes mellitus (DM) is a chronic complex metabolic disease characterized mainly by hyperglycemia. It appears to be a global epidemic and an increasingly major non-communicable disease, posing a threat to both affluent and nonaffluent societies. Diabetes dramatically increases the risk of developing stroke, chronic kidney disease, and coronary artery disease. These complications include chronic kidney disease, retinopathy, coronary artery disease, stroke, and diabetic foot ulcers. Compared to people without diabetes, adults with diabetes have a two to four times greater risk of dying from heart disease and stroke. This study aimed to assess the knowledge and attitude regarding complications among patients with diabetes visiting the Diabetes Clinics at King Fahad Specialist Hospital (KFSH) in the Qassim Region, Saudi Arabia. Methodology A cross-sectional study was conducted in which patients with DM who visited the Diabetic Clinics at KFSH from February 2023 to July 2023 were recruited using a nonprobability sampling technique. A validated questionnaire from previous research was used to collect data on the sociodemographic features of the participants and their knowledge regarding diabetic complications. Data were described using mean with standard deviation for continuous variables and proportion for categorical variables. Results A total of 368 patients were recruited. The majority of the respondents (144, 39.1%) had been living with diabetes for over 10 years, followed by 23.1% (85) of those with less than one year. Exactly 239 (64.9%) reported having a family history of diabetes. About 75% (276) of participants knew about diabetic complications, and 247 (67.1%) reported that their doctor had addressed them about diabetes complications. Among the sources of diabetes complication information, diabetologists were reported by 131 (35.6%) of the respondents, followed by 81 (22.0%) through social media, 65 (17.7%) from relatives, and 39 (10.6%) from family medicine doctors. On the other hand, the findings reveal that the different age groups have different levels of knowledge of hyperglycemia and hypoglycemia as diabetic complications (P = 0.031). However, there were no significant differences in the level of knowledge regarding complications between male and female patients (P > 0.05). Conclusions The study found that while the studied population had a relatively good knowledge of diabetic complications, some still lacked knowledge. The study also supports the need for individuals with a family history of diabetes to be aware of their risk and take steps for prevention, as a large proportion of participants reported having a family history of diabetes. Healthcare providers and diabetologists were the most frequent sources of information on diabetic complications. However, some also turned to social media. Focused education and awareness actions are crucial to ensure that people with diabetes have access to reliable information from various sources.