The mole concept is one of the fundamental concepts in chemistry. However, a number of studies carried out in various countries have shown that teachers and learners face many difficulties in teaching and learning the concept. One of the underlying sources of teaching and learning difficulties of the mole concept identified in literature is the gap in understanding the link between atomic theory and the mole concept. Therefore, the focus of this study was to assess teachers' conceptual understanding of the link between the atomic theory and the mole concept in selected secondary schools in Kitwe District in the Copperbelt Province of Zambia. The study was conducted in six secondary schools with a study sample of 30 chemistry teachers. Data were collected using the following multiple data collection strategies: questionnaire, lesson observations and focus group interviews. The study found that teachers understood how the atomic theory links to the mole concept in terms of the number (N) of atoms or formula units. They understood that a mole is equal to 6.02 x 10[superscript 23] atoms or formula units. However, their conceptual understanding of the link between relative atomic/formula mass and molar mass as the second linking idea between atomic theory and the mole concept was inadequate, because they did not fully understand relative atomic/formula unit mass and molar mass. The study recommended that professional development initiatives need to be organised with a focus on the updated definition of the mole and the key role of the unified atomic mass unit as the mass scale for microscopic entities in the understanding of the link between relative atomic/formula mass and molar mass. This is key for the conceptual understanding of the mole concept.