To investigate the knowledge, attitudes, and practices of diabetes among Saudi adults in Riyadh. Methods A questionnaire-based study was carried out in September 2019. A previously validated questionnaire was used to assess participants' knowledge. Results The study sample included 3,208 total participants. Of these, 53% were females and 47% were males. About 53.5% of the participants had good knowledge scores. The great majority of respondents did not know whether metformin could cause kidney damage (n = 2651, 82.6%) and more than half did not know whether long-term drug use could cause organ failure (n = 2073, 64.6%) and whether insulin could cause harmful effects (n = 1836, 57.2%). Results showed that 91.3% of the respondents stated that they would seek treatment if they or one of their family members got diabetes mellitus (DM). Approximately 50% of the participants (49.9%) regularly exercised. More than half (68%) of the respondents had never checked their blood glucose levels on an annual basis. More than half of the respondents tried to avoid refined sugar. Conclusion The majority of the participants had never checked their blood glucose levels. In addition, onethird of the participants believed that the use of complementary medicine could actually control diabetes.