A field experiment was conducted in the years 2012-2014, at the Plant Breeding Station in Polanowice near Krakow (220 m a.s.l.). The aim of the study was to investigate the effect of silicon (Si) on seed yield and quality of timothy-grass (Phleum pratense L.) of “Egida” cultivar. A univariate field experiment in randomized block design was repeated four times, and the area of the experimental plots was 10 m2. The soil on the experimental plots was a loess derived haplic phaeozem of bonitation class I. The experimental factor was spraying with a silicon formulation in the form of Optysil® fertilizer at three doses: 0.2, 0.5 and 0.8 dm3·ha−1. During the growing season, the plants were evaluated for their height, leaf greenness index (SPAD) and general condition. After harvesting, the seed yield and quality were assessed. The study revealed a significant effect of silicon on plant height, general condition and yield and quality of the seeds. The plants treated with silicon showed lower infestation rate with pathogens and pests than the control ones. Foliar fertilization with the highest dose of the silicon formulation (0.8 dm3·ha−1) caused a significant increase in seed yield as compared with control. The effects were also satisfactory in the plants treated with the formulation at 0.5 dm3·ha−1. The seeds obtained from silicon-treated plants were bigger, as revealed by the weight of 1000 seeds, and exhibited higher germination ability than the control seeds.