Elyounoussi scored in Basel's 2-1 second-leg win over Manchester City in the Champions League last 16 last season. The Swiss side lost the tie 5-2 on aggregate.
Southampton have signed attacking midfielder Mohamed Elyounoussi for a reported £16m from Basel.
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The 23-year-old Norway international, who scored 23 goals in 83 games in his last two seasons for the Swiss side, has agreed a five-year deal.
"I'm at my best when I use my creativity, so I think the club's philosophy will suit me," he said.
His arrival follows Dusan Tadic's £10m departure from the Saints to Ajax.
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More to follow.
Source: bbc.com