The production of milk is an essential branch of agricultural production in Poland. There have been considerable changes in the milk market in Poland over the last 20 years (such as, e.g., the adjustment of the market to the EU requirements, which had an impact on the functioning of this market as well as its spatial integration. This research mainly aimed to assess the processes of spatial integration on the Polish milk market in the period 1999–2018. In order to process the material collected, the author applied the analysis of price differences, the analysis of correlation, the Johansen test of cointegration and the Granger causality test. As a result of the research conducted, it was found that there is a long-term balance between prices in various voivodeships in Poland. Moreover, the closer the voivodeships were to one another, the greater the co-variability of prices was between them. In addition, it was indicated which voivodeships were crucial from the point of view of the process of revealing and determining raw milk prices in Poland by two distinguished periods (1999–2008 and 2009–2018).