Sustainable composite foams based on rice starch and cellulosic long microfibers were successfully fabricated using microwave irradiation. They were presented as a promising method to recycle some of the textile industry waste. A deep study of the processability and functionality of the composites revealed the performance improvement of starch with the addition of long cellulosic microfibers, especially with 6 wt.% of Arbocel ® , in terms of foamability, water, and mechanical resistance features. Moreover, sodium bicarbonate, which acted as a blowing and pulping agent, led to a lower density and better fiber distribution that resulted in an improvement of the foams' functionalities. The range of the study is new in the domain of long microfiber foam composites in terms of foaming capability, and mechanical, thermal, and water resistance properties. Furthermore, all foams showed excellent biodegradability properties against a fungus commonly found in the environment; for example, values around 60% weight loss after 33 days. Finally, the assessment of the CO2 emission during the process underlines the environmental benefits of the method employed.