Third doses of measles-mumps-rubella (MMR) vaccine have been administered during mumps outbreaks and in various non-outbreak settings. The immunogenicity of the rubella component has not been evaluated following receipt of a third dose of MMR vaccine. Young adults aged 18-31 years with documented two doses of MMR vaccine received a third dose of MMR vaccine between July 2009 and October 2010. Rubella neutralizing antibody titers were assessed before, 1 month, and 1 year after receipt of a third dose of MMR vaccine. Among 679 participants, 1.8% had rubella antibody titers less than 10 U/ml, immediately before vaccination, approximately 15 years after receipt of a second dose of MMR vaccine. One month after receipt of a third dose of MMR vaccine, average titers were 4.5 times higher and >50% of participants had a 4-fold boost. Response was highest among those with titers less than 10 U/ml prior to vaccination (geometric mean titer ratio = 18.8; 92% seroconversion) and decreased with increasing pre-vaccination titers. Average titers declined 1 year postvaccination but remained significantly higher than pre-vaccination levels. The proportion classified as low-positive antibody levels increased from 3% 1 month postvaccination to 24% 1 year postvaccination. Vaccination with a third dose of MMR vaccine resulted in a robust boosting of rubella neutralizing antibody response that remained elevated 1 year later. Young adults with low rubella titers are more likely to benefit from a third dose of MMR vaccine.