Background: Rheumatoid Arthritis (RA) is a systemic and chronic autoimmune disease with inflammation at the synovial joints. The purposes of this study were to evaluate the correlation between serological variables and temporomandibular disorders (TMDs) in patients diagnosed with RA, evaluated through diagnostic criteria for temporomandibular disorders (DC/TMD), and to check the influence of comorbidities on the different TMD signs and symptoms, or any serological variables. Methods: This observational cohort research study included seventeen patients affected by RA. The comorbidities and some variables from the initial serological analyses were collected from the clinical rheumatological charts. Then, the presence of any of the following TMD signs/symptoms, temporal myalgia, temporomandibular joint arthralgia, click or crepitus, was evaluated through the symptom questionnaire of the DC/TMD during clinical evaluation following the DC/TMD examination form. Results: Rheumatoid factor (RF), anticitrullinated protein antibodies (ACPA), and anti-nuclear antibody (ANA) positivity were present in 82.4%, 52.9%, and 41.2% of patients, respectively. Indicators of tissue inflammation were evaluated with median values of 21 mm/h for erythrocyte sedimentation rate (ESR) and 0.50 mg/dL for C-reactive protein (CRP). The RA patients presented comorbidities such as hypertension in 70.6% and diabetes in 23.5%. Negative ACPA had a significant association with myalgia (p = 0.03), and positive ANA was significantly associated with crepitus (p = 0.05). Conclusion: ANA and ACPA evaluation can be considered predictive serological tests associated with specific TMDs. Conversely, no influence of any comorbidities was found between different TMD signs and symptoms, or any serological variables.