Aromatic C-H activation in alkylarenes is a key step for the synthesis of functionalised organic molecules from simple hydrocarbon precursors. Known examples of such C-H activations often yield mixtures of products resulting from activation of least-hindered C-H bonds. Here we report highly selective ortho-C-H activation in alkylarenes by simple iridium complexes. We demon-strate that the capacity of the alkyl substituent to override the typical preference of metal-mediated C-H activation for least hindered aromatic C-H bonds results from transient insertion of iridium into the benzylic C-H bond. This enables fast iridium insertion into the ortho-C-H bond, followed by regeneration of the benzylic C-H bond by reductive elimination. Bulkier alkyl substituents increase ortho-selectivity. The described chemistry comprises a conceptually new alternative to existing approaches to aromatic C-H bond activation.