Teachers' imagination can help to foster students' understanding of scientific conceptions and stimulate them to generate creative ideas and inventions. Previous literature has indicated that individuals' scientific imagination may be influenced both by internal factors (e.g., personality and mental ability) and external environment. Based on the scientific imagination process model proposed by Ho, Cheng, Wang (2011), this study developed the Teacher Scientific Imagination Scale (TSIS) to assess elementary and high school teachers' imaginative ability in terms of three dimensions: personality, mental imagery and surroundings. Two samples of Taiwanese elementary and junior high school teachers participated in the development of the TSIS, one for item revision and the other for validation. Sample 1 was 890 teachers (245 males and 645 females) and sample 2 was 1500 teachers (549 males and 951 females). The Rasch partial credit model (PCM) was employed to assess model-data fit. The multidimensional Rasch partial credit model was employed to obtain more accurate estimation of correlations among the three subscales and to improve the reliability of person measures. Differential item functioning (DIF) analyses were conducted across genders. The results showed that the TSIS had a reasonably good model-data fit. The correlations of person measures from three subscales were between .66 and .71; the person separation reliabilities of the measures from the personality subscale, the mental Imagery scale, and the surroundings scale were .94, .90, and .92, respectively. No significant DIF was found across gender. The person measures from the three subscales were positively correlated with external variables: idea generation, self-efficacy, and creative achievement. The TSIS eventually consisted of 40 items in the three subscales: personality subscale (23 items), mental imagery subscale (five items), and surrounding subscale (12 items). It was concluded that the TSIS is a reliable and valuable self-report tool for assessing teachers' scientific imaginative ability, future research should address how the TSIS can be applied to science education, especially to the development of pre-service and in-service science teacher training courses.