Christopher Spivey, 51, was taken into custody on Wednesday morning as police confiscated computer equipment from his home in Rochford, Essex.
The tattoo artist claimed the death of the soldier in May last year was all an elaborate government hoax.
He told readers he had spent hours researching footage of his barbaric killing outside Woolwich Barracks in south west London and could “prove” it was set up by the UK establishment.
He is thought to have been arrested on suspicion of harassment via social media.
An Essex Police spokeswoman said: “A 51-year-old man from Rochford was arrested by Essex Police on suspicion of harassment in the early hours of Wednesday, July 30.
“He is at Southend Police Station helping officers with their enquires. The arrest relates to an allegation of harassment via social media which has been referred to Essex Police by Greater Manchester Police.”
Michael Adebolajo, 29, and Michael Adebowale, 23, were both given life sentences for killing Rigby in February.
The two ran down the 25-year-old with their car before inhumanely hacking him to death with cleavers and knives.
Adebolajo, who will spend the rest of his life in prison, had the first stage of an appeal rejected earlier this week.
His case will now be heard by the Court of Appeal in London.
No dates have been confirmed.
Adebowale received a minimum sentence of 45 years and was told last month he is now entitled to appeal his case.