The eukaryotic genome is organized within cells as chromatin. For proper information output, higher-order chromatin structures can be regulated dynamically. How such structures form and behave in various cellular processes remains unclear. Here, by combining super-resolution imaging (photoactivated localization microscopy PALM) and single-nucleosome tracking, we developed a nuclear imaging system to visualize the higher-order structures along with their dynamics in live mammalian cells. We demonstrated that nucleosomes form compact domains with a peak diameter of ∼160 nm and move coherently in live cells. The heterochromatin-rich regions showed more domains and less movement. With cell differentiation, the domains became more apparent, with reduced dynamics. Furthermore, various perturbation experiments indicated that they are organized by a combination of factors, including cohesin and nucleosome-nucleosome interactions. Notably, we observed the domains during mitosis, suggesting that they act as building blocks of chromosomes and may serve as information units throughout the cell cycle. Display omitted •We visualized chromatin structures and their dynamics in live mammalian cells•Nucleosomes form compact chromatin domains in live cells and move coherently•The domains are organized by nucleosome-nucleosome interactions and cohesin•The domains exist during mitosis and act as building blocks of chromosomes How a genome is organized and behaves in live cells remains unclear. Nozaki et al. visualized little bunches of chromatin, “chromatin domains,” and their dynamic behavior in live mammalian cells. The domains can work as “Lego blocks” of chromosomes to maintain genetic information throughout the cell cycle.