Species-rich semi-natural grasslands are declining all over northern Europe, and many plant species confined to such grasslands are currently under threat. We studied the development of populations of one such species, the field gentian Gentianella campestris, during three decades in the County of Södermanland, south of Stockholm, Sweden. Gentianella campestris is Red Listed as Endangered in Sweden. It is a strict biennial, and as far as known with only a transient seed bank. Large population fluctuations are a characteristic of this species, and its life history makes the species inherently sensitive to factors causing population reductions. We found that the number of sites with G. campestris has declined with over 60% in the last three decades. The total number of flowering individuals also show a strong decreasing trend, although there was an increase the last year (2020) at a few remaining sites. Cessation of grazing management is a major cause of the decline, but populations also disappeared from managed sites. It is possible that the management has been inappropriate, and circumstantial evidence suggests that summer drought might be an additional cause of population decline. Data from 2018, a year with an exceptional summer drought, supports this explanation. A sowing experiment indicated that recruitment of new populations is unlikely in the present-day landscape where most vegetation is unsuitable for G. campestris. Due to the poor prospects for long-term maintenance of grazing management in still remaining semi-natural grasslands, and the decline even at sites with current management, G. campestris faces a risk of becoming regionally extinct within the coming decades.