Joe Ritchie-Bennett, 39, was one of three people stabbed to death as they enjoyed the sunshine in Forbury Gardens on Saturday evening.
He was originally from Philadelphia, USA, but had been living in the UK for 15 years where he worked for a pharmaceutical company.
His father Robert Ritchie told CBS News: "The family is heartbroken they have lost their brilliant and loving son. This was senseless."
A friend wrote an emotional tribute to him on social media, saying he was a "wonderful and funny man".
Mr Ritchie-Bennett lost his husband of eight years to colon cancer in 2014, according to local media.
US ambassador to the UK Woody Johnson confirmed the 39-year-old was a US citizen and offered sympathies to his loved ones.
He tweeted: "I offer my deepest condolences to the families of those killed in the attack on 20 June. To our great sorrow, this includes an American citizen.
"Our thoughts are with all those affected. We condemn the attack absolutely and have offered our assistance to British law enforcement."
Secondary school teacher James Furlong, 36, was the first victim to be named.
He taught history, government and politics at The Holt School in Wokingham, eight miles from where he was stabbed, and was described as "beautiful" and "intelligent" by his parents.
Gary and Janet Furlong said in a statement: "He was the best son, brother, uncle and partner you could wish for. We are thankful for the memories he gave us all. We will never forget him and he will live in our hearts forever."
The suspect - believed to be 25-year-old Khairi Saadallah - is in custody after being detained on suspicion of murder and re-arrested under the Terrorism Act.
https://news.sky.com/story/reading-terror-attack-second-victim-was-39-year-old-american-man-12012334