Glyphosate resistance has evolved worldwide. Glyphosate is also the most used herbicide in Spain, and current changes in herbicide usage patterns can increase the risk of glyphosate resistance development. The objective of this study was to assess the glyphosate sensitivity of different selected weed species important in Spanish maize (Zea mays L.) fields. To this end, dose–response experiments were conducted under controlled conditions in a growth chamber to examine variation in glyphosate sensitivity among populations of five grass weed species and eight broadleaf weed species that are commonly found in the maize fields in Castilla y León, the biggest maize-growing region in Spain. The glyphosate doses that caused growth reduction by 50% (GR₅₀) were calculated for each weed population. No populations were resistant to glyphosate. In addition, baseline values of glyphosate sensitivity were determined for each weed species. The GR₅₀ baseline values ranged from 10.25 to 53.23 g ai ha^–1 for the dicotyledonous weed species and from 16.05 to 66.34 g ai ha^–1 for the monocotyledonous weed species. The ratio between the GR₅₀ values of the least and most sensitive populations was used to determine the SI₅₀ (sensitivity index at 50% growth reduction) for each weed species. The SI₅₀ values showed a 1.4- to 3.3-fold difference in sensitivity for dicotyledonous weed species and 1.4- to 2.4-fold difference for monocotyledonous weed species. The sensitivity index was also calculated as the ratio between the GR₅₀ values of the least sensitive population and the baseline GR₅₀ value estimated for a range of susceptible populations (SI₅₀b). SI₅₀b values showed a 1.2- to 1.6-fold difference in sensitivity for dicotyledonous weed species and 1.1- to 1.2-fold difference for monocotyledonous weed species. The sensitivity data generated in this study provide a reference for determining time-dependent changes in glyphosate sensitivity in the commonly found weeds in the maize fields of Castilla y Léon.