Patients with coronavirus disease 19 (COVID-19) are at high risk for thrombotic arterial and venous occlusions. At the same time, lung histopathology often reveals fibrin-based occlusion of small vessels in patients who succumb to the disease. Antiphospholipid syndrome (APS) is an acquired and potentially life-threatening thrombophilia in which patients develop pathogenic autoantibodies (aPL) targeting phospholipids and phospholipid-binding proteins. Small case series have recently detected aPL in patients with COVID-19. Here, we measured eight types of aPL (anticardiolipin IgG/IgM/IgA, anti-beta-2 glycoprotein I IgG/IgM/IgA, and anti- phosphatidylserine/prothrombin (PS/PT) IgG/IgM) in the sera of 172 patients hospitalized with COVID-19. We detected anticardiolipin IgM antibodies in 23%, anti-PS/PT IgG in 24%, and anti-PS/PT IgM in 18%. Any aPL was present in 52% of patients using the manufacturer's threshold and in 30% using a more stringent cutoff (>40 units). Higher levels of aPL were associated with neutrophil hyperactivity (including the release of neutrophil extracellular traps/NETs), higher platelet count, more severe respiratory disease, and lower glomerular filtration rates. Similar to patients with known and longstanding APS, IgG fractions isolated from patients with COVID-19 promoted NET release from control neutrophils. Furthermore, injection of these COVID-19 IgG fractions into mice accelerated venous thrombosis. Taken together, these studies suggest that a significant percentage of patients with COVID-19 become at least transiently positive for aPL and that these aPL are potentially pathogenic.