Summary Phosphate (Pi) starvation in plants induces dense and elongated root hairs, which increase the absorptive surface area of the roots and play a critical role in Pi uptake. The molecular mechanism underlying these changes remains unclear. Forward and reverse genetic approaches were employed to identify novel genes involved in root hair formation on Pi starvation. The mutant per2, with defects in root hair elongation specifically under low Pi conditions, was identified in a large‐scale genetic screen of T‐DNA insertion lines. The phenotype was caused by a mutation in the homeodomain protein ALFIN‐LIKE 6 (AL6). From a screen of mutants defective in genes that showed lower transcript abundance in per2 relative to wild‐type roots on low Pi medium, we identified four putative downstream targets of AL6, namely ETC1, NPC4, SQD2 and PS2, all of which were critical in root hair elongation of Pi‐deficient plants. The results further indicate that AL6 is involved in the control of growth and several key responses to Pi starvation. Our findings demonstrate that AL6 controls the transcription of a suite of genes critical for root hair elongation under low Pi conditions, suggesting a novel physiological function for an Alfin gene in Arabidopsis.