La Unión is situated in the Region of Murcia in the southeast of Spain. It has an area of 24.6 km², and had a population of 19,907 on 1 January 2018. It has an elevation of 86 m. Its average annual temperature is 17 °C. It has balmy winters. The sun shines 320 days per year. La Unión is situated in one of the sunniest areas in Europe; this kind of climate makes possible the many leisure activities, popular fiestas, sports, and cultural activities that are held in the town. The town is linked by a regular train to Cartagena which allows views of the past industrial heritage of the area and the more modern occupations of agriculture and tourism. La Unión lies within the built-up area of Cartagena and is surrounded on all landward sides by the City of Cartagena.