Strigolactones are a new group of plant hormones that play a pivotal role in the regulation of aboveground plant architecture. However, the mechanisms governing their regulation of plant growth and development are unknown. We characterized the effect of a synthetic strigolactone (GR24) on tomato (Solanum lycopersicon) roots and present evidence for its relationship with the plant hormone auxin. We demonstrate that strigolactones interfere with the inhibitory effect of exogenously applied auxin on root elongation. This GR24-induced root elongation is conveyed via an increase in root cell length accompanied by a reduction in cell diameter, and it occurs despite strigolactone’s reduction of cell division (detected as reduction of CYCB1;1 transcript). In addition, high concentrations of strigolactone lead to asymmetric root growth and inhibition of root-hair elongation. Exogenous application of NAA or IAA was unable to restore symmetric root growth and root-hair elongation in the presence of strigolactone. However, application of NPA, an auxin-efflux inhibitor, did restore root-hair elongation in the presence of strigolactone. Similarly, exogenous application of 2,4-D, a synthetic auxin that is not secreted by efflux carriers, restored root-hair elongation and symmetric growth in the presence of strigolactone. Nevertheless, 2,4-D was unable to prevent root elongation by strigolactones. Therefore, strigolactones’ effect on root growth and root-hair elongation appears to be mediated via an effect on auxin-efflux carriers. Nevertheless, more than one mechanism may govern strigolactones’ effect on root growth.