Palmer amaranth (Amaranthus palmeri S. Watson) is the most problematic weed in agronomic crop production fields in the United States. The objective of this study was to determine the effect of degree of water stress on the growth and fecundity of A. palmeri using soil moisture sensors under greenhouse conditions. Two A. palmeri biotypes collected from Nebraska were grown in loam soil maintained at 100%, 75%, 50%, 25%, and 12.5% soil field capacity (FC) corresponding to no, light, moderate, high, and severe water stress levels, respectively. Water was regularly added to pots based on soil moisture levels detected by Watermark or Decagon 5TM sensors to maintain the desired water stress level. Amaranthus palmeri plants maintained at ≤25% FC did not survive more than 35 d after transplanting. Amaranthus palmeri at 100%, 75%, and 50% FC produced similar numbers of leaves (588 to 670 plant^{− 1}) based on model estimates; however, plants at 100% FC achieved a maximum height of 178 cm compared with 124 and 88 cm at 75% and 50% FC, respectively. The growth index (1.1 × 10⁵ to 1.4 × 10⁵ cm³ plant ^{− 1}) and total leaf area (571 to 693 cm² plant ^{− 1}) were also similar at 100%, 75%, and 50% FC. Amaranthus palmeri produced similar root biomass (2.3 to 3 g plant ^{− 1}) at 100%, 75%, and 50% FC compared with 0.6 to 0.7 g plant ^{− 1} at 25% and 12.5% FC, respectively. Seed production was greatest (42,000 seeds plant ^{− 1}) at 100% FC compared with 75% and 50% FC (14,000 to 19,000 seeds plant ^{− 1}); however, the cumulative seed germination was similar (38% to 46%) when mother plants were exposed to ≥50% FC. The results of this study show that A. palmeri can survive ≥50% FC continuous water stress conditions and can produce a significant number of seeds with no effect of on seed germination.