Branson Riley Carlisle died on 23 November just hours after he was bitten on the back in his home in Albertville, Alabama.
The little boy was immediately taken to hospital but medics were unable to save him, WHNT reported.
His mother Jessica Carlisle told Alabama.com: “He said something had stung him. It was a brand new pyjama top and this spider was in his shirt. I saw a welt on his back.”
Carlisle had been bitten by a brown recluse herself several years ago and knew she had to act fast, catching the spider and rushing her son to hospital.
She says medics became concerned the venom might be “going systemic”, a rare and more dangerous outcome of a bite.
The site of the bite itself swelled up to the size of a baseball, with black, blue and red marks at the bite site and “was all knotted up under the skin”.
Despite the best efforts of hospital staff, Branson’s organs began to bleed and his heart stopped.
Carlisle told WGNTV: “The only time they got his heartbeat back was the seventeenth time they shocked him and they had his heart beating for four seconds, and I had enough time to tell him I loved him.”
She added: “I feel very blessed to have had him, and I’m not going to question it, because it was meant to be.”
While the family grieves for their loss, they say they will pay tribute to Branson by carrying out his Christmas wish – to donate his toys to needy children.
The boy’s church is currently accepting toy donations in his honour at the Free Life Worship Centre in Boaz, Alabama.
The brown recluse has a venomous bite, though typically only does so when disturbed.
Reactions to the venom vary with some experiencing no symptoms at all, to chills, fever, nausea, sweating and ulcers or simply a small red mark which heals quickly.
In extreme cases coma, seizures and kidney failure can occur, the US National Library of Medicine reports.
For those with higher sensitivity levels, a small white blister appears at the bite site soon after the bite. The tissue may become hard. Lesions are dry, blue-gray or blue-white patches with ragged edges surrounded by redness. This color pattern has yielded the nickname "red, white and blue," and, in severe reactions, the bite site can develop a "volcano lesion," according to The Ohio State University. The damaged tissue becomes gangrenous and leaves an open wound that can be as large as a human hand. It can take eight weeks or longer for full recovery, and scars may result.
According to Alabama.com, Regional Poison Control Centre director Ann Slattery has not heard of a single death from a brown recluse bit in her 32 years on the job.
Carlisle says her son was in normal health and that the house was visited by pest controllers once a month.