The Vietnam stock market is a new market in which dividend taxes are higher than capital gains taxes, Vietnamese firms have a high state-ownership, a majority of dividends are paid by cash and there is significant inconsistency in dividend payouts. This paper investigates the effect of dividend increase announcements on stock price reactions. A total of 198 announcements for 101 companies listed on the HOSE market was investigated, the results indicated that announcements of dividend increase do not cause reactions in security prices. The obtained results support Black’s tax-based dividend signaling hypothesis about the condition for dividend announcements becoming market information when dividend taxes are higher than capital gains taxes. These findings of this study are also in line with other previous studies.