In a number of animal species, reproductive isolation has repeatedly and independently evolved between populations adapted to contrasting but not to similar environments. This process is commonly known as parallel speciation, and examples in plants are enigmatically rare. Here, we show that natural selection has repeatedly and independently driven the evolution of reproductive isolation between the coastal ecotypes of the groundsel Senecio lautus, a herbaceous plant found in Australia. We found that in crosses between populations separated by half a million years of divergence, crosses within the same ecotype showed greater fertility than crosses between different ecotypes. This pattern of reproductive isolation was similar but weaker when crossing recently diverged populations of these ecotypes. Unexpectedly, molecular estimates of gene flow between parapatric populations were nil, suggesting that despite being largely interfertile, parapatric populations from different ecotypes must have experienced little gene flow in the recent past. These results suggest that in Senecio, ecological adaptation facilitates the evolution of both extrinsic and intrinsic reproductive isolation across a complex geographic landscape and leads to the repeated and independent origin of multiple plant species in parapatry.