We studied the upper 0–1000 mm of the bottom sediment core from the underwater Sofia Lepneva Ridge of Lake Teletskoye. The core sediments accumulated with a rate of 0.3 mm/year (with regard to their humidity, with a rate of 0.45 mm/year) and have an age of about 2000 years. A total of 194 species (212 varieties and forms) of diatoms have been revealed. Analysis of diatom composition in tanatocoenoses showed prevalence of species inhabiting the bottom and periphyton, widely distributed geographically, indifferent to salinity, and preferring a weakly alkaline environment, as well as beta-mesosaprobionts. The number of diatom valves ranged from 0.86 to 64.4 mln valves/g, with an average of 22.90 ± 0.78 mln valves/g. Over the last millennium, the peaks of the abundance of diatoms, especially representatives of the cold-water assemblage, including the main dominant Aulacoseira subarctica (O. Müll.) Haw., are referred to the periods with low water temperature. The pH value for the period from 172 BC to 2006 AD has been reconstructed from the proportion of the numbers of valves of pH-indicating diatom species, which varied from 7.51 to 7.69, with an average of 7.580 ± 0.003. The pH value predicted up to 2259 AD was estimated by spectral (Fourier) analysis at 7.53–7.63, with a similar average of 7.580 ± 0.001. Analysis of reconstructed and predicted pH values showed their insignificant changes and correspondence to a weakly alkaline environment (7.0–8.5). These pH values suggest stability of the lake environment both in the past periods and in the nearest future.