This study, the first such attempt in Turkey, aimed to investigate the distribution of diatom species related to the variation of total phosphorus (TP) in eight basins to determine trophic weight and indicator values of diatom species and to develop the Trophic Index-Turkey (TIT) for assessing the ecological status of freshwaters. The distribution of the diatom species in 225 running water bodies from 8 basins of Turkey was determined in 2014 (summer and fall) and 2015 (spring and summer) in association with TP variation. Trophic values for 219 species were determined during this study and many of these are presented for the first time. Optima and tolerance TP values were calculated for each species in the different aquatic ecosystems using weighted average regression. Diatom species such as Clevamphora ovalis, Caloneis amphisbaena, Gyrosigma acuminatum, Navicula tripunctata, and Nitzschia umbonata showed high trophic weight values, while Achnanthidium minutissimum, Cymbella excisa, Didymosphenia geminata, Hannaea arcus, and Meridion circulare had low trophic weight values. These findings provide supplementary information about the biotic integrity of the water bodies and could be used to develop the TIT for the bioassessment of freshwater ecosystems, especially running water bodies.