Pectin is a dietary fiber, and its health effects have been described extensively. Although there are limited clinical studies, there is a growing body of evidence from in vitro studies investigating the effect of pectin on human gut microbiota. This comprehensive review summarizes the findings of gut microbiota modulation in vitro as assessed by 16S rRNA gene-based technologies and elucidates the potential structure-activity relationships. Generally, pectic substrates are slowly but completely fermented, with a greater production of acetate compared with other fibers. Their fermentation, either directly or by cross-feeding interactions, results in the increased abundances of gut bacterial communities such as the family of Ruminococcaceae, the Bacteroides and Lachnospira genera, and species such as Lachnospira eligens and Faecalibacterium prausnitzii, where the specific stimulation of Lachnospira and L. eligens is unique to pectic substrates. Furthermore, the degree of methyl esterification, the homogalacturonan-to-rhamnogalacturonan ratio, and the molecular weight are the most influential structural factors on the gut microbiota. The latter particularly influences the growth of Bifidobacterium spp. The prebiotic potential of pectin targeting specific gut bacteria beneficial for human health and well-being still needs to be confirmed in humans, including the relationship between its structural features and activity.