After regaining independence in 1918, activists of the Polish Tourist Society (PTK) – Stanisław Karpowicz, Fr. Andrzej Wyrzykowski, Józef Pietraszewski, and Zdzisław Lenartowicz – initiated the creation of a museum. On May 5, 1921, a dynamically developing institution was opened without its own premises; collections (archaeological, natural, numismatics, folk culture) were stored in private homes of members. In 1925, they were moved to the PTK building on Gołębińska Street (now Żeromskiego). Leon Wilkoński cataloged exhibits from 1927, including the "princely" grave discovered in 1928 in Krakówka.

The collections were enriched with donations from residents, excavations (e.g., Lenartowicz, Wilkoński), and treasures like 346 denarii from Gnieszowice (1934, 135 coins survived). By 1933, 6,000 exhibits were gathered, including unique folk costumes (a sukmana survived); the exhibition in two rooms attracted visitors. The outbreak of World War II in 1939 brought disaster: destroyed natural objects, depleted numismatics and ethnography; archaeology survived best, becoming the basis for post-war reactivation.

By order of the Minister of Culture on April 18, 1956, the Museum in Sandomierz was established (from June 1), with its seat in the restored Oleśnicki House. The attic was adapted (1960–61); the first exhibition on Mickiewicz, Friends Circle for youth. In 1970, the Historical-Artistic and Archaeological-Ethnographic departments were separated. Despite a modest staff, the collection of exhibits, exhibitions, and lectures continued, building on the surviving regional resources.

The administrative reform of 1975 granted the status of a District Museum (order of February 3, 1976), which initiated dynamic development: new departments (Archaeology, Ethnography, History, Art, Photographic Documentation, Scientific-Educational), strengthened staff. In 1977, the National Memory Farm in Wiktoryn was opened (liquidated in 1991). In 1978, rooms in the town hall were acquired for historical-artistic and photographic exhibitions about Sandomierz.

In 1980, the J. Iwaszkiewicz Literature Branch was established in the cathedral house, collecting memorabilia of writers from the region. In 1986, the Museum of Armed Struggle in Janów Lubelski was created (independent in 1993) and the Castle was handed over for use – after 30 years of renovation, it became the base for exhibitions, warehouses, and administration. This period marked intense enrichment of educational and exhibition offerings.

In 1998, stabilization of the castle hill, in 2001 reconstruction; the Oleśnicki House was handed over. In 2002, the Literature Department was moved to the Castle and the attic was modernized; the cathedral building was handed over. Renovation of the tower and knight's hall (2004–05). In 2003, the Sybilla award for conservation; the Castle became the sole seat after 2013, with permanent and temporary exhibitions, conferences, and cultural events.

Entry into the State Register of Museums (1999) confirmed the high level. The museum, heir to the tradition from 1921, collects regional collections, conducts research, education (lessons, workshops), publications, and popularization. Post-war directors (e.g., J. Waszkiewicz 1978–95, Z. Czubowa 1995–2009) oversaw development; the Castle integrates the past with the present as a center of exhibitions and meetings.