Constraints on the seaweed culture in Indonesia until now still rely on seeds obtained by vegetative propagation and have poor quality, which often causes a decrease in genetic variability. Planting seaweed seedlings from tissue cultures that have been acclimatized in a greenhouse laboratory is believed to improve the quality of the seaweed seeds. This research aimed to observe the growth of tissue culture acclimatized Eucheuma cottonii seaweed seedlings to different rearing times in greenhouses applied to floating raft cages. This study used a completely randomized design (CRD) with three treatments and three replications. Acclimatization treatment was conducted for 2 weeks, 4 weeks, and 6 weeks. Seaweed seedlings acclimatized in the greenhouse are reared on bamboo floating rafts in the sea, with an initial weight of 5 grams each. The length of maintenance at sea is 28 days. The resulting daily growth rate in two-week acclimatization was 2.92%, the four-week treatment was 2.93%, and the six-week treatment was 2.57%. in raising seedlings at sea, the highest daily growth rate value was in the four-week treatment of 7.25%, followed by the two-week treatment of 7.11% and the six-week treatment of 6.43%.