Transforming growth factor β-1 (TGF-β) acts as both a tumour suppressor and a tumour promoter in a context-dependent manner. The tumour-promoting activities of TGF-β are likely to result from a combination of Smad and non-Smad signalling pathways but remain poorly understood. Here we show that TGF-β-mediated activation of RhoA is dependent on the kinase activity of ALK5 and that continuous ALK5 activity maintains basal RhoA–ROCK signalling, cell morphology and actin dynamics in serum-starved rodent fibroblasts independently of Smad2, Smad3 and Smad4. In immortalized human diploid fibroblasts, we show that oncogenic rewiring by transduction of V12 HaRas instigates regulation of RhoA–ROCK signalling through an autocrine TGF-β1–ALK5 pathway. Furthermore, we show that ALK5-mediated activation of RhoA is required for efficient V12 HaRas, V-Raf and V600E BRAF transformation and V12 HaRas-mediated anchorage-independent growth. These findings identify a new pro-oncogenic activity of TGF-β and indicate that tumours harbouring V12 HaRas and V600E BRAF mutations may be susceptible to TGF-β signalling inhibitors.