This systematic review and meta-analysis aimed to evaluate thrombocytopenia as a prognostic biomarker in patients with COVID-19. We performed a systematic literature search using PubMed, Embase, and EuropePMC. The main outcome was composite poor outcome, a composite of mortality, severity, need for intensive care unit care, and invasive mechanical ventilation. There were 8963 patients from 23 studies. Thrombocytopenia occurred in 18% of the patients. Male gender (p=0.037) significantly reduce the incidence. Thrombocytopenia was associated with composite poor outcome (RR 1.90 [1.43, 2.52], p<0.001; I2: 92.3%). Subgroup analysis showed that thrombocytopenia was associated with mortality (RR 2.34 [1.23, 4.45], p<0.001; I2: 96.8%) and severity (RR 1.61 [1.33, 1.96], p<0.001; I2: 62.4%). Subgroup analysis for cut-off <100 × 109/L showed RR of 1.93 [1.37, 2.72], p<0.001; I2: 83.2%). Thrombocytopenia had a sensitivity of 0.26 (0.18, 0.36), specificity of 0.89 (0.84, 0.92), positive likelihood ratio of 2.3 (1.6, 3.2), negative likelihood ratio of 0.83 (0.75, 0.93), diagnostic odds ratio of 3 (2, 4), and area under curve of 0.70 (0.66 - 0.74) for composite poor outcome. Meta-regression analysis showed that the association between thrombocytopenia and poor outcome did not vary significantly with age, male, lymphocyte, d-dimer, hypertension, diabetes, and CKD. Fagan's nomogram showed that the posterior probability of poor outcome was 50% in patients with thrombocytopenia, and 26% in those without thrombocytopenia. The Deek's funnel plot was relatively symmetrical and the quantitative asymmetry test was non-significant (p = 0.14). This study indicates that thrombocytopenia was associated with poor outcome in patients with COVID-19. © 2021 Cambridge University Press. All rights reserved.