There is an unofficial prayer area near to Jalan Kayu The Prata Cafe (#B1-19) in Fusionopolis.

From One North MRT (CC23), 
1. Go to exit C which leads you to B2 of Fusionopolis One
2. Take the escalator up to level B1 and turn left. 
3. Look for the toilet sign and walk towards it (on your left), you will see quite a few halal restaurants along the way. 
4. The toilets are just opposite Jalan Kayu The Prata Cafe (#B1-19) where you can perform you ablution. 
5. Once done, walk towards Jalan Kayu The Prata Cafe (#B1-19), you will see an Exit sign on the left, leading to a grey door 'Stair G'. Push the door leading to 'Stair G' and you will be at the unofficial prayer area which is shared for muslimah and muslimin. 
There is a prayer mat and qibla sign available. Please bring your own telekung. 

UPDATE: 06/2024: the musollah beside Streats is temporarily closed due to the preparation for the Harry Potter: Forbidden Forest Experience. 

There is a separate musollah for muslimah and muslimin available at RWS (The Forum, Level B1) near to The Streats. 
Prayer mats and telekung are available. Qibla sign and ablution area provided as well. 

Contributed by Brother Jeffri

There is another musollah which can fit 2 people near to the Casino entrance, and there's a tap to take wudu inside. The musollah for men and women are separate (women's one is just opposite the men's one). Sejadah and qiblat sign available. No slippers provided since wudu place is inside the musollah. 

Contributed by Brother Rauzan

Please note that the unofficial prayer area at level 4R, near to the library no longer exists. 

There is a new unofficial prayer area at the staircase on level 3 of the carpark.

From the Serangoon MRT (CC13/NE12),
1. Take Exit G which will lead you to Level B2 of NEX.
2. Take the lift/escalator up to Level 3 of the NEX mall and walk towards the Fairprice Xtra (#03-42) Exit.  
3. On the left of the Fairprice Xtra (#03-42) Exit is the connecting bridge leading to the Carpark Level 4.
4. Walk down the bridge and turn right and walk till the end of the Carpark (near to the trolley bay). Please be careful of the vehicles. 
5. You will see a ramp leading down to Level 3 Carpark with more trolleys. Walk down the ramp and on the right of lot 195 is the staircase with sign 'Smoke Stop Lobby Staircase 10'. The unofficial prayer area is located just behind that door and shared for both muslimah and muslimin. 

There is a Qibla sign and a prayer mat available. Please bring your own telekung. 
For ablution, you would have to go to the nearest toilet in NEX mall before making your way to the unofficial prayer area.

The address of the restaurant is: #01-07 HometeamNS Balestier. 
There is a musollah in the restaurant, please note that this is only for customers. For more information, the musollah can be found in this facebook video at timestamp 2:17.

There is an ablution area inside the prayer room. Separate prayer room for men and women. No air-con, only fan. Please bring your own telekung. Prayer mats are provided.  
For assistance, please ask the staff.

Contributed by Sister Nisaa