Seed and root hair protective protein (SRPP) is expressed in seeds and root hairs, localized in the cell wall, and involved in cell wall integrity. We analyzed a loss-of-function mutant of SRPP, focusing on siliques and seeds. The srpp-1 plants generated dark brown shrunken seeds at a high rate. The germination rate of these defect seeds of srpp-1 was less than 6%, although apparently normal srpp-1 seeds germinated at a rate of 83%. The production ratio of severe phenotypic seeds was dependent on the growth conditions. When the srpp-1 plants were cultivated at low humidity, the defect ratio was 73%, which was significantly higher than that at normal humidity. Defects of the silique and seeds could be detected on day 7 after pollination and the apical region of the siliques displayed a severe phenotype at a high frequency. Complementation with an SRPP gene under the control of promoters specific to the embryo, seed coat, or valve (carpel) partially rescued the phenotype, and complementation using the SRPP promoter fully rescued the phenotype. Furthermore, overexpression of SRPP enhanced the thermotolerance. After the treatment of seeds at 50 °C for 2 h, the germination rate of the seeds from overexpression with the 35S promoter increased to levels twice that of the wild-type seeds. Under the same conditions, no srpp-1 seeds germinated. These results indicate that SRPP is essential for the production of normal viable seeds in siliques under stress conditions. It is possible that modification of the SRPP gene improves seed integrity.