Trisodium borate is a chemical compound of sodium, boron, and oxygen, with formula Na3BO3, or (Na+)3[BO3]3−.

[3] It is a sodium salt of the orthoboric acid B(OH)3.

However, "sodium orthoborate" has been used also for a compound with formula Na4B2O5, which would correspond to an equimolar mixture of sodium metaborate NaBO2 and trisodium borate proper.

Sodium carbonate Na2CO3 will react with sodium metaborate NaBO2 or boric oxide B2O3 to form the orthoborate and carbon dioxide when heated between 600 and 850 °C:[3] Difficult to obtain in pure form from melts.

[5] When dissolved in water, the orthoborate anion partially hydrolyzes into metaborate [BO2]− and hydroxide OH−:[3] Electrolysis of a solution of sodium orthoborate generates sodium perborate at the anode.