Polysaccharides guar galactomannan (guar gum), locust bean galactomannan (locust bean gum) and tamarind galactoxyloglucan were selectively oxidized by galactose oxidase. The degrees of oxidation of the products were 18–28% for guar galactomannan, 10–16% for locust bean galactomannan and 12–14% for tamarind galactoxyloglucan, calculated from the ratio of oxidized galactose units and total carbohydrates. The rheological properties of the unoxidized and oxidized polysaccharide solutions were investigated by determining their viscosities, storage and loss moduli, and temperature dependence of moduli from 20 °C to 90 °C. All the studied oxidized polysaccharides formed hydrogels throughout the entire temperature range. Concentration (0.2–1% w/v) and degree of oxidation had an effect on the gel formation. The oxidized galactomannans formed stable gels already in low concentrations, such as 0.2–0.4% w/v, while oxidized galactoxyloglucan required a concentration of 0.8% w/v to be stable up to 90 °C. The oxidized polysaccharide hydrogels are highly potential materials for food and medical applications requiring thermal stability.