The aim of this study was to investigate potential differences in liver protein expression of Atlantic salmon fed genetically modified (GM) Roundup Ready soy at a high inclusion level (25% inclusion, constituting 21% of crude protein in the diet) for 7 months or a compositionally similar non-GM diet. The liver was selected as the target organ due to its importance in the general metabolism, and 2D gel electrophoresis used as a screening tool. Samples from 12 individual fish from each diet group were evaluated. Of a total of 781 analysed protein spots, only 36 were significantly different by ANOVA (p < 0.05) in abundance between the diet groups. All these spots had low fold differences (1.2-1.6) and high false discovery rate (q = 0.44), indicating minor differences in liver protein synthesis between fish fed GM and non-GM soy. Additionally, low fold differences were observed. Four protein spots were analyzed by liquid chromatography tandem mass spectrometry and identified using a combination of online searches in NCBI and searches in an inhouse database containing salmonid expressed sequence tags and contigs. Follow-up on these proteins by real-time polymerase chain reaction did not identify differences at the transcriptional level.