The plant hormone auxin is a central regulator of plant development. In Arabidopsis, the PINOID (PID) protein serine/threonine kinase is a key component in the signaling of this phytohormone. To further investigate the biological function of PID, we performed a screen for PID-interacting proteins using the yeast two-hybrid system. Here, we show that PID interacts with two calcium-binding proteins: TOUCH3 (TCH3), a calmodulin-related protein, and PID-BINDING PROTEIN 1 (PBP1), a previously uncharacterized protein containing putative EF-hand calcium-binding motifs. The interaction between PID and the calcium-binding proteins is significant because it is calcium dependent and requires an intact PID protein. Furthermore, the expression of all three genes (PID, TCH3, and PBP1) is up-regulated by auxin. TCH3 and PBP1 are not targets for phosphorylation by PID, suggesting that these proteins act upstream of PID. PBP1 was found to stimulate the autophosphorylation activity of PID, and calcium influx and calmodulin inhibitors where found to enhance the activity of PID in vivo. Our results indicate that TCH3 and PBP1 interact with the PID protein kinase and regulate the activity of this protein in response to changes in calcium levels. This work provides the first molecular evidence for the involvement of calcium in auxin-regulated plant development.