To determine the prevalence of clonal T-large granular lymphocyte (T-LGL) cells in patients with spondyloarthritis (SpA) and psoriatic arthritis (PsA) and to define possible risk factors for this condition. We present a cross-sectional analysis with retrospective and prospective aspects. 115 SpA patients, 48 PsA patients and 51 controls were recruited between December 28, 2017 and January 23, 2019. Flow cytometry (FACS) was performed to screen for aberrant T-LGL cells. Molecular analysis was then employed to confirm the diagnosis in patients with suggestive FACS findings. Patients with clonal T-LGL populations were followed prospectively by FACS analysis. Electronic patient files were retrospectively analyzed to determine risk factors. Median age was 49 years for SpA, 55.5 years for PsA, and 54 years for controls. Median disease duration of SpA and PsA was 15 years and 11 years, respectively. 79.8% of patients had received biologics at some point, 75.5% had ever received tumor necrosis factor (TNF) inhibitors. 59.5% were treated with TNF inhibitors at the time of study inclusion. We identified clonal T-LGL expansions in 13 individuals equaling a prevalence of 6% (13/214). T-LGL patients were taking TNF inhibitors more frequently at the time of study inclusion (p = 0.022) and were more likely to have ever been treated with TNF inhibition (p = 0.046). Clonal T-LGL expansions can be detected in patients with SpA, PsA and also in healthy controls. Confirming earlier results, exposure to TNFα-blocking agents appears to increase the risk of developing clonal expansions of T-LGL cells.