Background/Objectives: Schoolteachers play a central role in shaping their students’ beliefs and attitudes towards oral health. Our aim was to investigate the oral and periodontal health knowledge of schoolteachers in Riyadh and factors affecting this knowledge. Methods: Government schoolteachers from representative areas of Riyadh (n = 895) responded to a structured questionnaire. Descriptive statistics, t-tests, one-way analysis of variance, and multiple linear regression (p ≤ 0.05). Results: Teachers demonstrated good basic oral/periodontal health knowledge (mean score = 60.21 ± 10.68). Most knew that toothbrushing is necessary to preserve dental (78.66%) and periodontal (57.88%) health; that gingival bleeding (74.41%), swelling (64.25%), and abscess formation (52.96%) are signs of periodontal disease; about 63% identified dental biofilm as an etiologic factor, and 58% knew that periodontitis may cause gingival recession and influence systemic health (74.07%). However, only 38% knew that dental flossing is necessary to preserve periodontal health, and 66.03% believed that gingival health can be restored with a special toothpaste. Teachers who were female, older in age, worked in north Riyadh, and taught the intermediate stage demonstrated statistically significantly better knowledge than the other categories. Conclusions: The studied sample of schoolteachers possesses acceptable basic oral health knowledge but has inadequate knowledge of periodontal health. Factors influencing teachers’ knowledge were age, gender, region of work, and teaching stage.