Fibrillin-1 (FBN1) is the major component of extracellular matrix microfibrils, which are required for proper development of elastic tissues including heart and lung. Through protein-protein interactions with latent TGF-beta binding protein 1 (LTBP1), microfibrils assist regulation of TGF-beta signaling. Mutations within the 47 epidermal growth factor-like (EGF) repeats of FBN1 cause autosomal dominant disorders including Marfan Syndrome that disrupt TGF-beta signaling. We recently identified 2 novel protein O-glucosyltransferases, Protein O-glucosyltransferase 2 (POGLUT2) and Protein O-glucosyltransferase 3 (POGLUT3), that modify a few EGF repeats on Notch. Here, using mass spectral analysis, we show that POGLUT2 and POGLUT3 also modify over half of the EGF repeats on FBN1, fibrillin-2 (FBN2), and LTBP1. While most sites are modified by both enzymes, some sites show a preference for either POGLUT2 or POGLUT3. POGLUT2 and POGLUT3 are homologs of POGLUT1, which stabilizes Notch proteins by addition of O-glucose to Notch EGF repeats. Like POGLUT1, POGLUT2 and 3 can discern a folded versus unfolded EGF repeat, suggesting POGLUT2 and 3 are involved in a protein folding pathway. In vitro secretion assays using recombinant FBN1 revealed reduced FBN1 secretion in POGLUT2 knockout, POGLUT3 knockout, and POGLUT2 and 3 double knockout HEK293T cells compared to wild type. These results illustrate that POGLUT2 and 3 function together to O-glycosylate protein targets, and that these modifications play a role in secretion of target proteins.