The objective of this study was to detect plant protein adulterated in fluid milk using nano‐high‐performance liquid chromatography (HPLC)–tandem mass spectroscopy (LC‐MS/MS) combined with proteomics. Unadulterated milk and samples adulterated with soy protein, pea protein, hydrolyzed wheat protein, and hydrolyzed rice protein were prepared, with plant protein level ranged from 0.5% to 8% in total protein. Sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS‐PAGE) gels clearly revealed that centrifugation at 20,000 g for 60 min would reduce band intensity of casein and albumin in milk. Results of nano‐HPLC‐MS/MS indicated the major proteins of soy (β‐conglycinin, glycinin), pea (vincilin, convicilin, legumin), and wheat (glutenin and gliadin) in adulterated milks, allowing detection of soy protein and hydrolyzed wheat protein at the level above 0.5% in total protein and pea protein at the level of 2 and 4%. No rice protein was identified in milk samples adulterated with hydrolyzed rice protein. Combined with principal component analysis, nano‐HPLC‐MS/MS could discriminate all the adulterated samples from authentic milk. This study demonstrated the feasibility of nano‐HPLC‐MS/MS on the detection of (hydrolyzed) plant protein adulterated in milk.