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Changes in plant phenology are a direct indicator of climate change and can produce important consequences for agricultural and ecological systems. This study analyzes changes in plant phenology in the 1961–2010 period (for both the entire interval and in three successive multi-decades: 1961–1990, 1971–2000 and 1981–2010) in southern and southeastern Romania, the country’s most important agricultural region. The analysis is based on mean monthly air temperature values collected from 24 regional weather stations, which were used for extracting the length (number of days) of phenophases (growing season onset, budding–leafing, flowering, fruiting, maturing, dissemination of seeds, start of leaf loss, end of leaf loss) and of the overall climatic growing season (CGS, which includes all phenophases), by means of the histophenogram method. Using a number of reliable statistical tools (Mann–Kendall test, Sen’s slope estimator and the regression method) for exploring annual trends and net (total) changes in the length of the phenological periods, as well as for detecting the climate—growing season statistical relationships, our results revealed complex phenology changes and a strong response in phenological dynamics to climate warming. Essentially, a lengthening of all phenophases (maximal in the maturing period, in terms of statistical significance and magnitude of trends—on average 0.48 days/yr/24 days net change in the 1961–2010 period, or even 0.94 days/yr/28 days net change in the 1971–2000 sub-period) was noticed, except for the fruiting and dissemination phenophases, which were dominated by negative trends in the number of days, but partially statistically significant (at a confidence level threshold of at least 90%). The CGS exhibited overall increasing trends, with an average of 0.21 days/yr/11 days net change in the 1961–2010 interval, and even of 0.90 days/yr/27 days net change in the 1981–2010 sub-period. Moreover, based on the slope values obtained upon application of a linear regression to mean temperature and CGS, we discovered that a 1 °C increase in climate warming accounted for a remarkable lengthening of the CGS, on average of 14 days between 1961 and 2010, and of 16 days between 1981 and 2010. Our results can help improve the adaptation of agroecological systems to future climate change.