The right-back officially departed Fratton Park on July 1 after allowing his two-year deal to expire.
Kenny Jackett was keen to keep the tough-tackling defender but Thompson shelved contract negotiations in February.
And the Blues’ failure to be promoted from League One meant he departed to try to achieve his dream of playing in the second tier.
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After no previous links or reports of Thompson trialling with a club, he lined up for Bristol City under-23s in their pre-season friendly against Yeovil on Saturday.
The defender played the first half in the young Robins’ 2-1 defeat against the National League side, being replaced by Opi Edwards on 46 minutes.
Boss Lee Johnson is ‘having a look’ at the 28-year-old, with City short of right full-back options.
Yet news of Thompson’s pursuit of winning an Ashton Gate deal has left sections of supporters up in arms.
Robins supporters haven’t forgotten the Chester-born man’s actions from the best part of five years ago when he was captain of Swindon.
On November 15, 2014, City travelled to the County Ground for a crunch League One promotion battle.
Bristol sat top of the table, with Swindon seven points adrift in third.
The encounter was heated from kick-off, with City forward Wade Elliott being sent off for the visitors after just two minutes for clashing with home defender Jack Stephens.
Michael Smith’s 78th-minute strike was the only goal of the game, delivering the hosts a 1-0 win.
And that effort prompted Thompson to celebrate in front of the travelling fans – something they vividly remember.
Taking to Twitter, several members of the City faithful have pleaded with their club not to sign the defender on a permanent basis.
Here’s how some reacted...
We better not sign Nathan Thompson.
Nathan Thompson can do one.
Don’t sign Nathan Thompson, cheers @BristolCity.
Nathan Thompson is absolutely not welcome at our club @LeeJohnsonCoach to sign him would be an insult to all our fans he disrespected.
If Nathan Thompson gets anywhere near the first team squad and a @BristolCity shirt, I call on all supporters to boo him back to where he came from. @LeeJohnsonCoach what are you thinking?
Praying that Nathan Thompson is only here on trial or training with us and is not signing permanently, definitely a character I don’t want near my club.