High fiber diets are known to have many benefits to sows (e.g., improving reproductive performance such as vitality of piglets, stimulating gut health, and improving well-being), and due to their fermentation properties, they may have different functions in lactating sows. Fast fermentable fiber favors the growth of beneficial microbiota which contributes to improved gut health, and slow fermentable fiber reduces constipation and provides a delayed release of some nutrients which benefits the farrowing process of sows. On the other hand, high fiber diets may reduce feed intake and nutrient digestibility. The objective of this study was to investigate the effects of dietary fiber concentration and fermentation rate on lactating sow and litter performance, nutrient digestibility, milk composition, immunity, and fecal microbiota composition. Four diets were formulated based on these two factors, with diets 1 and 2 formulated at low (15%) total dietary fiber (TDF) and diets 3 and 4 at high (25%) TDF, while diets 1 and 3 included with a high level of slow fermentable fiber and diets 2 and 4 included a high level of fast fermentable fiber. The proportion of fast fermentable fiber in 4 treatments was 1.25%, 2.5%, 3.75%, and 5%, respectively. The main fiber sources were sugar beet pulp, oat hull, wheat millrun, and barley. Effective energy (EE), crude protein (CP), and lysine levels were comparable among each treatment group. A total of 136 lactating sows (Landrace × Large White) were stratified by parity, body weight (BW), and back fat (BF) and then randomly assigned to each treatment. Feed allowance was set to increase from 3 kg/d (primiparous sow) or 3.5 kg/d (multiparous sow) at farrowing to a maximum of 7 or 8 kg/d at 8 d or 9 d post-farrowing. Results showed that sows in high fiber groups had a lower frequency of stillbirth and constipation, accompanied by decreased apparent total tract digestibility (ATTD) of nutrients and increased litter weight gain and body fat loss (P < 0.05). Sows in fast fermentable fiber groups showed a decreased BW and protein loss (P < 0.05), lower interleukin-8 (IL-8) concentration in serum (P < 0.05), higher ATTD of gross energy (GE) (P = 0.016), day 19 dry matter (DM) (P < 0.001), CP (P < 0.001) and nitrogen (P < 0.001), and a tendency to have increased lactose levels in milk (P = 0.099). The interaction between the fiber level and fermentation rate had significant effects on ATTD of day 2 and day 19 DM (P < 0.001), CP (P < 0.001), and nitrogen (P < 0.001), and tended to have effects on ATTD of GE (P = 0.092) and serum interleukin-10 (IL-10) concentration (P = 0.083). High fiber diets had a decreased diversity of microbiota, but the proportion of health-related members of the family Prevotellaceae, Ruminococcaceae, and Lachnospiraceae was significantly increased. In conclusion, the inclusion of high dietary fiber (DF) in lactation diets may offer advantageous effects on reproductive performance and gut health without negatively impacting factors such as feed intake, colostrum and milk composition, microbiota composition, and sow immunity. Moreover, fast fermentable fiber decreases IL-8 levels, BW loss, and protein loss in lactating sows. The results suggest that when formulating diets for lactating sows, it may be beneficial to consider high levels of DF with a fast rate of fermentability.