Vascular plants have a network of vasculature in their leaves, which supplies water and nutrients and exports photoassimilates to other tissues. The vascular network is patterned during the development of leaf primordia through the induction of provascular differentiation by auxin. Arabidopsis thaliana Dof5.8, encoding a Dof-type transcription factor, is expressed early in provascular cells under the control of the MONOPTEROS transcription factor, also known as auxin response factor 5 (ARF5). Here, we report the effect of overexpressing Dof5.8 in provascular cells on the formation of the vascular network. Overexpression of Dof5.8 inhibited the formation of higher-order veins in cotyledons and leaves, probably through transcriptional repression by Dof5.8. The expression of auxin-associated transcription factor genes, DORNRöSCHEN and SHI-RELATED SEQUENCE 5, was downregulated in the Dof5.8 overexpressors, and overexpression of these genes partially rescued the impaired formation of higher-order veins in Dof5.8-overexpressing lines, suggesting that the overexpression of Dof5.8 modulates the auxin response and leads to impaired vein formation in A. thaliana.