The high-mobility group protein B1 (HMGB1) belongs to alarmins — a group of molecules involved in inflammatory responses. HMGB1 is actively studied in the context of certain rheumatic diseases (RDs), including systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), ankylosing spondylitis (AS), and psoriatic arthritis (PsA), but the accumulated knowledge remains insufficient. HMGB1 can also act as an antigen, yet the level of anti-HMGB1 antibodies is poorly understood in RDs. The aim of this study was to investigate the concentrations of HMGB1 and anti-HMGB1 antibodies in four RDs. Using enzyme-linked immunosorbent assay (ELISA), plasma samples from patients with RA (n = 60), AS (n = 60), SLE (n = 24), PsA (n = 30), and healthy donors (HD) (n = 60) were analyzed. After adjustment for age and disease duration, it was shown that the concentration of HMGB1 was significantly increased in SLE (p < 0.01), RA (p < 0.01), and AS (p = 0.017), while a statistically non-significant increase in HMGB1 was observed in PsA (p = 0.07) compared to HD. Among the four diseases, the highest level of HMGB1 was found in SLE (p < 0.01). The concentration of anti-HMGB1 antibodies was also elevated in SLE (p < 0.01), RA (p = 0.026), and AS (p = 0.028). Using correlation and regression analysis, a strong direct association was established between the level of HMGB1 and the DAS28 index in RA (p < 0.01 for both analyses). The results of the study describe characteristic changes in HMGB1 and anti-HMGB1 antibody levels in RDs and indicate the involvement of HMGB1 in the pathogenesis of these diseases.