Aquatic invasive species are recognized as a global threat to conservation of native species and a cost to society. To develop effective suppression and monitoring programs for invasive species, fisheries managers require accurate, affordable, and efficient tools for invasive species detection. In the U.S. Pacific Northwest, the rapid expansion of invasive Esox lucius (northern pike) poses threats to native species as well as the viability of tribal, sport, and commercial fisheries. To help monitor changes in the distribution of this species, we developed and rigorously field‐tested an environmental DNA (eDNA) assay to detect E. lucius. The assay successfully amplified tissue-derived DNA of E. lucius from 36 locations east and west of the Continental Divide and did not amplify DNA of over 40 nontarget species. This assay was then used to assist with monitoring the distribution of invasive E. lucius in the upper Columbia River basin in Washington and Idaho. Sixty‐two eDNA samples were collected at 35 locations of known and unknown E. lucius presence. Two samples per site (one on each bank) were collected in larger waterbodies. E. lucius eDNA detections were consistent with previous observations of live fish during angler and gill‐net surveys, confirming the reliability of the eDNA assay. At two of the 35 sites, only one of the paired samples was positive for E. lucius DNA. Varying results between opposite bank samples highlight the need for increased sampling effort when the target species are at low abundance and in large waterbodies. The eDNA assay described in this paper can be used by managers to identify the presence of E. lucius, monitor their expansion in western North America, and guide E. lucius suppression projects.