Lake Ilmen is one of the most productive lakes in the northwestern part of European Russia. The purpose of the study was to assess the trophic status of the Ilmen in the period 2003–2020 based on two different indicators. The trophic status index (ITS) is calculated from two hydrochemical indicators: the pH value and the percentage of water saturation with oxygen. The other indicator of the trophic status is the average annual concentration of total phosphorus. Interannual ITS values for the period from 2003 to 2020 were determined. The input for the calculations was data from lake monitoring regularly conducted by the North-Western Department of Hydrometeorology and Environmental Monitoring. It has been established that the trophic status of Lake Ilmen in the specified period was mainly characterized as eutrophic except for years 2006,
2008, 2017, and 2019, when it was mesotrophic. According to the concentration of total phosphorus in 2003, the trophic status of the lake was characterized as hypertrophic, in 2018 as mesotrophic, while in other years it was eutrophic. It is advisable to apply the results of the study in decision-making for reducing the anthropogenic phosphorus loading on the lake.