定向选择(Directional selection)是天择的一种形式。

对于一种特征(性状)的不同状态而言，因为不同状态带给生物体的适应度(适存度, fitness)有差异，其中一种状态的适应度较高，具有此状态的有利个体就能产生较多的子代，造成此状态的等位基因(allele)便倾向于在族群中增加，并渐渐取代其他等位基因，此过程即称为定向选择。

假设，影响一特征/性状的等位基因有A1与A2两种，如果同型合子A1A1的适应度最高，而异型合子A1A2适应度中等，A2A2适应度最低（ω11>ω12>ω22, ω代表适应度），在天择的影响下，保留A1、并淘汰A2的过程就是定向选择。若环境维持稳定，A1基因所代表的性状持续带给此物种较高的适应度，「有利的」A1基因随着时间而渐渐散布到整个族群，基因频率提高，最后达到稳固(fixation)。

In population genetics, directional selection is a mode of natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype. Under directional selection, the advantageous allele increases as a consequence of differences in survival and reproduction among different phenotypes. The increases are independent of the dominance of the allele, and even if the allele is recessive, it will eventually become fixed.