Abstract Truncated O-GalNAc glycosylation is an important feature of pancreatic ductal adenocarcinomas (PDAC) and expression of truncated O-GalNAc glycans is strongly associated with decreased survival and poor prognosis. It has been proven, that aberrant O-GalNAc glycosylation influence PDAC signaling to promote oncogenic properties, but elucidation of the influence of truncated O-GalNAc glycosylation on different signaling molecules has just been started. We herein elucidated the impact of aberrant O-GalNAc glycosylation on two important PDAC signaling pathways, namely AKT/mTOR and RAS/MAPK. In PDAC cells expressing truncated O-GalNAc glycans, we identified differentially expressed proteins associated with AKT/mTOR and RAS/MAPK pathways using quantitative proteomics. Since AKT, a key-signaling molecule in PDAC, was among the identified proteins, we analyzed AKT and found a strikingly enhanced S473 phosphorylation and identified a previously unknown O-GalNAc-modification. Consecutive analysis of COSMC knockdowns in PDAC revealed strong effects on AKT upstream and downstream effector molecules. Interestingly, truncated O-GalNAc glycans could facilitate an mTORC1 inhibitor resistance using AZD8055. In addition, as AKT/mTOR pathway has extensive cross talks with RAS/MAPK pathway we analyzed the pathways and found it negatively regulated. Finally, we found that the expression of epithelial-mesenchymal-transition markers, key features of aggressive PDACs cells, are enhanced and truncated O-GalNAc glycans enhance pancreatic cancer cell growth in a xenograft mouse model. Our study demonstrates that truncated O-GalNAc glycans have a strong impact on AKT/mTOR and RAS/MAPK signaling pathways, are modulated by EGF or IGF-1 signaling and should be considered for targeted therapy of these pathways in PDAC.