Mitogen-activated dual-specificity MAPK phosphatases are important negative regulatorsin the MAPK signalling pathways responsible for many essential processes in plants. In ascreen for mutants with reduced organ size we have identified a mutation in the active siteof the dual-specificity MAPK phosphatase INDOLE-3-BUTYRIC ACID-RESPONSE5(IBR5) that we named tinkerbell (tink) due to its small size. Analysis of the tink mutant indicatesthat IBR5 acts as a novel regulator of organ size that changes the rate of growth inpetals and leaves. Organ size and shape regulation by IBR5 acts independently of the KLUgrowth-regulatory pathway. Microarray analysis of tink/ibr5-6 mutants identified a likely rolefor this phosphatase in male gametophyte development. We show that IBR5 may influencethe size and shape of petals through auxin and TCP growth regulatory pathways.