Janssen is one of Europe's most exciting young forwards, having scored 27 goals in 34 matches in the Eredivisie last season.
A transfer fee is yet to be agreed between the clubs, but AZ have allowed the 21-year-old to hold talks with Spurs and he arrived in London on Tuesday.
Several teams have expressed an interest in Janssen, who already has five caps for Holland, but Mauricio Pochettino's side are the first he has spoken to this summer.
AZ are believed to want around £12million for their top goalscorer, a fee unlikely to deter Spurs, although negotiations are ongoing around additional performance-related payments.
Tottenham are desperate to add forward cover for Harry Kane, who played in all 38 of his team's Barclays Premier League matches last term.
Kane, who won the Golden Boot award with 25 goals, is unlikely to be dislodged from the first team, but another offensive player would give Pochettino greater options off the bench and in cup competitions.
Janssen has enjoyed a remarkable rise to stardom, joining AZ only last summer after he scored 19 times for Almere City in the Dutch second tier.
Renowned for his high energy and excellent technique, Janssen then exploded into form this calendar year with 21 goals in 17 league appearances.
He also found the net against England in Holland's 2-1 win at Wembley in March
Source : PA
Source: PA