The aim was to analyze the psychometric properties of the Spanish version of the Gain in Alzheimer Care Instrument (GAIN), providing validity evidence based on its internal structure, reliability, item analysis, and relationships with other variables. A sample of 113 informal caregivers of people with dementia completed the GAIN, along with questionnaires assessing burden, general mental health, stress, anxiety, depression, and life satisfaction. Confirmatory factor analysis showed a single-factor structure with adequate fit indices. Reliability of GAIN scores was satisfactory, with McDonald’s omega equal to .91. Items yielded adequate homogeneity indices. Validity evidence based on relationships with other variables was provided by positive correlations between GAIN scores and life satisfaction, and negative correlations with burden, general mental health problems, stress, anxiety, and depression. All these correlations were statistically significant, and most of them were of moderate magnitude. The Spanish version of the GAIN has a single-factor structure and satisfactory psychometric properties. It is quick and easy to apply and given the association between GAIN scores and other variables, it may be used to provide information about a caregiver’s psychological health status.