The current research as aimed (i) to isolate and select the purple nonsulfur bacteria (PNSB) possessing the potassium-solubilizing ability from acid paddy fields and (ii) to evaluate the ability to release the plant growth-promoting substances (PGPS) of selected PNSB. A total of 35 acid sulfate (AS) soil samples were collected in An Giang province, Vietnam. Then, 70 PNSB strains were isolated from the AS soil samples. In the current study, the isolated strains were screened and selected according to their tolerability to acidic conditions, ability to solubilize potassium, and characteristics of a plant growth promoter on basic isolation media with various incubation conditions. Therein, three strains, TT07.4, AN05.1, and AC04.1, presented the highest potassium solubilization under the microaerobic light (11.8–17.7 mg L−1) and aerobic dark (16.4–24.7 mg L−1) conditions and stresses from Al3+, Fe2+, and Mn2+ toxicity. The selected strains were identified as Rhodopseudomonas pentothenatexigens by the 16S rDNA sequence, with 99% similarity. The selected acidic-resistant strains possessed the traits of biofertilizers under both microaerobic light and aerobic dark conditions, with abilities to fix nitrogen (0.17–6.24; 7.93–11.2 mg L−1); solubilize phosphorus from insoluble compounds with 3.22–49.9 and 9.49–11.2 mg L−1 for Al-P, 21.9–25.8 and 20.2–25.1 mg L−1 for Ca-P, and 10.1–29.8 and 18.9–23.2 mg L−1 for Fe-P; produce 5-aminolevulinic acid (0.63–3.01; 1.19–6.39 mg L−1), exopolymeric substances (0.14–0.76; 0.21–0.86 mg L−1), indole-3-acetic acid (12.9–32.6; 13.6–17.8 mg L−1), and siderophores (28.4–30.3; 6.15–10.3%). The selected potassium-solubilizing strains have a great potential to apply in liquid form into rice seed and solid form in AS soils to supply nutrients and PGPS for enhancing rice growth and grain yield.