Siam Commercial Bank's profit of 11.4 billion baht ($365.27 million) for the three months ended in March compared to 11.9 billion baht in the same period a year earlier. That missed the average 11.7 billion baht estimate in a Reuters survey of three analysts.
The decline in profits was due to marketing expenses relating to getting more customers to use mobile and digital banking services, it said in a statement.
Last month, the country's largest banks said they would waive fees for digital transactions, a major source of non-interest income as customers turned to non-banking alternatives for payments and transfers.
SCB's operating expenses rose 20.6 percent from a year earlier.
The bank was focused on "speed and high security of our digital banking platform," chief executive, Arthid Nanthawithaya, said in a statement.
Mobile banking will be a source of future business opportunities, Arthid said.
Earlier this year, Siam Commercial Bank said it would reduce headcount and branches and shift its focus to digital banking.