A young girl has died from injuries sustained in Sunday's bomb blast in Bangkok, bringing the death toll from the attack to three.
The apparent grenade blast took place near an anti-government protest site in the heart of the capital.
A woman, 59, and a four-year-old boy were confirmed dead on Sunday. Doctors said on Monday that the little boy's sister died later of brain injuries.
Twenty-two people were hurt, including a nine-year-old boy in intensive care.
Sunday's attack came hours after gunmen opened fire on an anti-government rally in eastern Thailand, killing a five-year-old girl.
Thai Prime Minister Yingluck Shinawatra has condemned the attacks, describing them as "terrorist acts for political gain".
UN Secretary Ban Ki-moon has also spoken out, calling for violence "from any quarter" to end immediately.
Tensions across Thailand have escalated since a wave of anti-government protests began in November.
The demonstrators want Yingluck to resign to make way for an appointed interim government, but she has refused.
Last week, several people were killed in clashes that erupted in Bangkok when police began clearing sites blocked by protesters.
No group has so far said they carried out either of the weekend attacks.
But the BBC's Jonathan Head in Bangkok says it appears to be the start of retaliation by the armed wing of the so-called "red-shirt" movement that backs the governing Pheu Thai party.
Yingluck heads a government that won elections in 2011 with broad support from rural areas.
In response to the protests, the prime minister called snap elections on 2 February, which her government was widely expected to win.
However, the polls were boycotted by the opposition and voting was disrupted by protesters at about 10 percent of polling stations, meaning by-elections are needed before a government can be formed.
Correspondents say red-shirt activists have watched with growing frustration as the protesters - who enjoy the backing of the military and powerful royalists - have been allowed to obstruct the government and sabotage the election.
On Sunday, leaders of the pro-government United Front for Democracy against Dictatorship (UDD) held a rally attended by thousands of supporters.
"This fight will be harder than any other ... You must think how we can deal with [protest leader] Suthep [Thaugsuban] and those supporting him," UDD leader and Pheu Thai official Jatuporn Prompan said.