Chinook salmon (Oncorhynchus tshawytscha) are external fertilizers that display sneak-guard alternative reproductive tactics. The larger hooknose males dominate mating positions, while the smaller jack males utilize sneak tactics to achieve fertilization. Although poorly understood, previous studies have suggested that differences in spermatozoa quality may play a critical role in sperm competition. Considering that the seminal plasma strongly regulates spermatozoa quality and other processes critical for fertilization success, we employed label free quantitative mass spectrometry utilizing ion mobility separation coupled to cross-species bioinformatics to examine the seminal plasma proteome of Chinook salmon. A total of 345 proteins were identified in all biological replicates analyzed, including many established seminal plasma proteins that may serve as future biomarkers for Chinook salmon fertility and sperm competition. Moreover, we elucidated statistically significant protein abundance differences between hooknose and jack male tactics. Proteins involved in membrane remodeling, proteolysis, hormonal transport, redox regulation, immunomodulation, and ATP metabolism were among the proteins reproducibly identified at different levels and represent putative factors influencing sperm competition between jack and hooknose males. This study represents the largest seminal plasma proteome from teleost fish and the first reported for Chinook salmon. Significance: Chinook salmon (Oncorhynchus tshawytscha) males represent an example of male alternative reproductive tactics where diverse reproductive strategies are thought to increase sexual selection. While seminal plasma has been shown to play an important regulatory role in sperm competition in many species, little is known about the protein composition of the seminal plasma of salmon. Therefore, seminal plasma isolated from the two alternative reproductive tactics of Chinook salmon (small sneaky jacks and large dominant hooknoses) were analyzed by label free quantitative mass spectrometry employing data independent acquisition and ion mobility separation. This yielded the largest proteome data set of the seminal plasma from salmon and the first to examine protein abundance differences between male alternative reproductive tactics. The quantitative proteomic data provides insight into possible unique mechanistic aspects of Chinook salmon alternative reproductive tactics utilized for sperm competition and fertilization success. (C) 2017 Elsevier B.V. All rights reserved.